The Variation of Animals and Plants under Domestication/XXVII

CHAPTER XXVII.

PROVISIONAL HYPOTHESIS OF PANGENESIS.


 * PRELIMINARY REMARKS.—FIRST PART:—THE FACTS TO BE CONNECTED UNDER A SINGLE POINT OF VIEW, NAMELY, THE VARIOUS KINDS OF REPRODUCTION—THE DIRECT ACTION OF THE MALE ELEMENT ON THE FEMALE—DEVELOPMENT—THE FUNCTIONAL INDEPENDENCE OF THE ELEMENTS OR UNITS OF THE BODY—VARIABILITY—INHERITANCE—REVERSION.


 * SECOND PART:—STATEMENT OF THE HYPOTHESIS—HOW FAR THE NECESSARY ASSUMPTIONS ARE IMPROBABLE—EXPLANATION BY AID OF THE HYPOTHESIS OF THE SEVERAL CLASSES OF FACTS SPECIFIED IN THE FIRST PART—CONCLUSION.

In the previous chapters large classes of facts, such as those bearing on bud-variation, the various forms of inheritance, the causes and laws of variation, have been discussed; and it is obvious that these subjects, as well as the several modes of reproduction, stand in some sort of relation to each other. I have been led, or rather forced, to form a view which to a certain extent connects these facts by a tangible method. Every one would wish to explain to himself, even in an imperfect manner, how it is possible for a character possessed by some remote ancestor suddenly to reappear in the offspring; how the effects of increased or decreased use of a limb can be transmitted to the child; how the male sexual element can act not solely on the ovule, but occasionally on the mother-form; how a limb can be reproduced on the exact line of amputation, with neither too much nor too little added; how the various modes of reproduction are connected, and so forth. I am aware that my view is merely a provisional hypothesis or speculation; but until a better one be advanced, it may be serviceable by bringing together a multitude of facts which are at present left disconnected by any efficient cause. As Whewell, the historian of the inductive sciences, remarks:—"Hypotheses may often be of service to science, when they involve a certain portion of incompleteness, and even of error." Under this point of view I venture to advance the hypothesis of Pangenesis, which implies that the whole organisation, in the sense of every separate atom or unit, reproduces itself. Hence ovules and pollen-grains,—the fertilised seed or egg, as well as buds,—include and consist of a multitude of germs thrown off from each separate atom of the organism.

In the First Part I will enumerate as briefly as I can the groups of facts which seem to demand connection; but certain subjects, not hitherto discussed, must be treated at disproportionate length. In the Second Part the hypothesis will be given; and we shall see, after considering how far the necessary assumptions are in themselves improbable, whether it serves to bring under a single point of view the various facts.

Reproduction may be divided into two main classes, namely, sexual and asexual. The latter is effected in many ways—by gemmation, that is by the formation of buds of various kinds, and by fissiparous generation, that is by spontaneous or artificial division. It is notorious that some of the lower animals, when cut into many pieces, reproduce so many perfect individuals: Lyonnet cut a Nais or freshwater worm into nearly forty pieces, and these all reproduced perfect animals. It is probable that segmentation could be carried much further in some of the protozoa, and with some of the lowest plants each cell will reproduce the parent-form. Johannes Müller thought that there was an important distinction between gemmation and fission; for in the latter case the divided portion, however small, is more perfectly organised; but most physiologists are now convinced that the two processes are essentially alike. Prof. Huxley remarks, "fission is little more than a peculiar mode of budding," and Prof. H. J. Clark, who has especially attended to this subject, shows in detail that there is sometimes "a compromise between self-division and budding." When a limb is amputated, or when the whole body is bisected, the cut extremities are said to bud forth; and as the papilla, which is first formed, consists of undeveloped cellular tissue like that forming an ordinary bud, the expression is apparently correct. We see the connection of the two processes in another way; for Trembley observed that with the hydra the reproduction of the head after amputation was checked as soon as the animal began to bud.

Between the production, by fissiparous generation, of two or more complete individuals, and the repair of even a very slight injury, we have, as remarked in a former chapter, so perfect and insensible a gradation, that it is impossible to doubt that they are connected processes. Between the power which repairs a trifling injury in any part, and the power which previously "was occupied in its maintenance by the continued mutation of its particles," there cannot be any great difference; and we may follow Mr. Paget in believing them to be the selfsame power. As at each stage of growth an amputated part is replaced by one in the same state of development, we must likewise follow Mr. Paget in admitting "that the powers of development from the embryo are identical with those exercised for the restoration from injuries: in other words, that the powers are the same by which perfection is first achieved, and by which, when lost, it is recovered." Finally, we may conclude that the several forms of gemmation, and of fissiparous generation, the repair of injuries, the maintenance of each part in its proper state, and the growth or progressive development of the whole structure of the embryo, are all essentially the results of one and the same great power.

Sexual Generation.—The union of the two sexual elements seems to make a broad distinction between sexual and asexual reproduction. But the well-ascertained cases of Parthenogenesis prove that the distinction is not really so great as it at first appears; for ovules occasionally, and even in some cases frequently, become developed into perfect beings, without the concourse of the male element. J. Müller and others admit that ovules and buds have the same essential nature. Certain bodies, which during their early development cannot be distinguished by any external character from true ovules, nevertheless must be classed as buds, for though formed within the ovarium they are incapable of fertilisation. This is the case with the germ-balls of the Cecidomyide larvæ, as described by Leuckart. Ovules and the male element, before they become united, have, like buds, an independent existence. Both have the power of transmitting every single character possessed by the parent-form. We see this clearly when hybrids are paired inter se, for the characters of either grandparent often reappear, either perfectly or by segments, in the progeny. It is an error to suppose that the male transmits certain characters and the female other characters; though no doubt, from unknown causes, one sex sometimes has a stronger power of transmission than the other.

It has been maintained by some authors that a bud differs essentially from a fertilised germ, by always reproducing the perfect character of the parent-stock; whilst fertilised germs become developed into beings which differ, in a greater or less degree, from each other and from their parents. But there is no such broad distinction as this. In the eleventh chapter, numerous cases were given showing that buds occasionally grow into plants having new and strongly marked characters; and varieties thus produced can be propagated for a length of time by buds, and occasionally by seed. Nevertheless, it must be admitted that beings produced sexually are much more liable to vary than those produced asexually; and of this fact a partial explanation will hereafter be attempted. The variability in both cases is determined by the same general causes, and is governed by the same laws. Hence new varieties arising from buds cannot be distinguished from those arising from seed. Although bud-varieties usually retain their character during successive bud-generations, yet they occasionally revert, even after a long series of bud-generations, to their former character. This tendency to reversion in buds is one of the most remarkable of the several points of agreement between the offspring from bud and seminal reproduction.

There is, however, one difference between beings produced sexually and asexually, which is very general. The former usually pass in the course of their development from a lower to a higher grade, as we see in the metamorphoses of insects and in the concealed metamorphoses of the vertebrata; but this passage from a lower to a higher grade cannot be considered as a necessary accompaniment of sexual reproduction, for hardly anything of the kind occurs in the development of Aphis amongst insects, or with certain crustaceans, cephalopods, or with any of the higher vascular plants. Animals propagated asexually by buds or fission are on the other hand never known to undergo a retrogressive metamorphosis; that is, they do not first sink to a lower, before passing on to their higher and final stage of development. But during the act of asexual production or subsequently to it, they often advance in organisation, as we see in the many cases of "alternate generation." In thus speaking of alternate generation, I follow those naturalists who look at the process as essentially one of internal budding or of fissiparous generation. Some of the lower plants, however, such as mosses and certain algæ, according to Dr. L. Radlkofer, when propagated asexually, do undergo a retrogressive metamorphosis. We can to a certain extent understand, as far as the final cause is concerned, why beings propagated by buds should so rarely retrogress during development; for with each organism the structure acquired at each stage of development must be adapted to its peculiar habits. Now, with beings produced by gemmation,—and this, differently from sexual reproduction, may occur at any period of growth,—if there were places for the support of many individuals at some one stage of development, the simplest plan would be that they should be multiplied by gemmation at that stage, and not that they should first retrograde in their development to an earlier or simpler structure, which might not be fitted for the surrounding conditions.

From the several foregoing considerations we may conclude that the difference between sexual and asexual generation is not nearly so great as it at first appears; and we have already seen that there is the closest agreement between gemmation, fissiparous generation, the repair of injuries, and ordinary growth or development. The capacity of fertilisation by the male element seems to be the chief distinction between an ovule and a bud; and this capacity is not invariably brought into action, as in the cases of parthenogenetic reproduction. We are here naturally led to inquire what the final cause can be of the necessity in ordinary generation for the concourse of the two sexual elements.

Seeds and ova are often highly serviceable as the means of disseminating plants and animals, and of preserving them during one or more seasons in a dormant state; but unimpregnated seeds or ova, and detached buds, would be equally serviceable for both purposes. We can, however, indicate two important advantages gained by the concourse of the two sexes, or rather of two individuals belonging to opposite sexes; for, as I have shown in a former chapter, the structure of every organism appears to be especially adapted for the concurrence, at least occasionally, of two individuals. In nearly the same manner as it is admitted by naturalists that hybridism, from inducing sterility, is of service in keeping the forms of life distinct and fitted for their proper places; so, when species are rendered highly variable by changed conditions of life, the free intercrossing of the varying individuals will tend to keep each form fitted for its proper place in nature; and crossing can be effected only by sexual generation, but whether the end thus gained is of sufficient importance to account for the first origin of sexual intercourse is very doubtful. Secondly, I have shown, from the consideration of a large body of facts, that, as a slight change in the conditions of life is beneficial to each creature, so, in an analogous manner, is the change effected in the germ by sexual union with a distinct individual; and I have been led, from observing the many widely-extended provisions throughout nature for this purpose, and from the greater vigour of crossed organisms of all kinds, as proved by direct experiments, as well as from the evil effects of close interbreeding when long continued, to believe that the advantage thus gained is very great. Besides these two important ends, there may, of course, be others, as yet unknown to us, gained by the concourse of the two sexes.

Why the germ, which before impregnation undergoes a certain amount of development, ceases to progress and perishes, unless it be acted on by the male element; and why conversely the male element, which is enabled to keep alive for even four or five years within the spermatheca of a female insect, likewise perishes, unless it acts on or unites with the germ, are questions which cannot be answered with any certainty. It is, however, possible that both sexual elements perish, unless brought into union, simply from including too little formative matter for independent existence and development; for certainly they do not in ordinary cases differ in their power of giving character to the embryo. This view of the importance of the quantity of formative matter seems probable from the following considerations. There is no reason to suspect that the spermatozoa or pollen-grains of the same individual animal or plant differ from each other; yet Quatrefages has shown in the case of the Teredo, as did formerly Prevost and Dumas with other animals, that more than one spermatozoon is requisite to fertilise an ovule. This has likewise been clearly proved by Newport, who adds the important fact, established by numerous experiments, that, when a very small number of spermatozoa are applied to the ova of Batrachians, they are only partially impregnated and the embryo is never fully developed: the first step, however, towards development, namely, the partial segmentation of the yelk, does occur to a greater or less extent, but is never completed up to granulation. The rate of the segmentation is likewise determined by the number of the spermatozoa. With respect to plants, nearly the same results were obtained by Kölreuter and Gärtner. This last careful observer found, after making successive trials on a Malva with more and more pollen-grains, that even thirty grains did not fertilise a single seed; but when forty grains were applied to the stigma, a few seeds of small size were formed. The pollen-grains of Mirabilis are extraordinarily large, and the ovarium contains only a single ovule; and these circumstances led Naudin to make the following interesting experiments: a flower was fertilised by three grains and succeeded perfectly; twelve flowers were fertilised by two grains, and seventeen flowers by a single grain, and of these one flower alone in each lot perfected its seed; and it deserves especial notice that the plants produced by these two seeds never attained their proper dimensions, and bore flowers of remarkably small size. From these facts we clearly see that the quantity of the peculiar formative matter which is contained within the spermatozoa and pollen-grains is an all-important element in the act of fertilisation, not only in the full development of the seed, but in the vigour of the plant produced from such seed. We see something of the same kind in certain cases of parthenogenesis, that is, when the male element is wholly excluded; for M. Jourdan found that, out of about 58,000 eggs laid by unimpregnated silk-moths, many passed through their early embryonic stages, showing that they were capable of self-development, but only twenty-nine out of the whole number produced caterpillars. Therefore it is not an improbable view that deficient bulk or quantity in the formative matter, contained within the sexual elements, is the main cause of their not having the capacity of prolonged separate existence and development. The belief that it is the function of the spermatozoa to communicate life to the ovule seems a strange one, seeing that the unimpregnated ovule is already alive and continues for a considerable time alive. We shall hereafter see that it is probable that the sexual elements, or possibly only the female element, include certain primordial cells, that is, such as have undergone no differentiation, and which are not present in an active state in buds.

Graft-hybrids.—When discussing in the eleventh chapter the curious case of the Cytisus adami, facts were given which render it to a certain degree probable, in accordance with the belief of some distinguished botanists, that, when the tissues of two plants belonging to distinct species or varieties are intimately united, buds are afterwards occasionally produced which, like hybrids, combine the characters of the two united forms. It is certain that when trees with variegated leaves are grafted or budded on a common stock, the latter sometimes produces buds bearing variegated leaves; but this may perhaps be looked at as a case of inoculated disease. Should it ever be proved that hybridised buds can be formed by the union of two distinct vegetative tissues, the essential identity of sexual and asexual reproduction would be shown in the most interesting manner; for the power of combining in the offspring the characters of both parents, is the most striking of all the functions of sexual generation.

Direct Action of the Male Element on the Female.—In the chapter just referred to, I have given abundant proofs that foreign pollen occasionally affects the mother-plant in a direct manner. Thus, when Gallesio fertilised an orange-flower with pollen from the lemon, the fruit bore stripes of perfectly characterised lemon-peel: with peas, several observers have seen the colour of the seed-coats and even of the pod directly affected by the pollen of a distinct variety; so it has been with the fruit of the apple, which consists of the modified calyx and upper part of the flower-stalk. These parts in ordinary cases are wholly formed by the mother-plant. We here see the male element affecting and hybridising not that part which it is properly adapted to affect, namely the ovule, but the partially developed tissues of a distinct individual. We are thus brought half-way towards a graft-hybrid, in which the cellular tissue of one form, instead of its pollen, is believed to hybridise the tissues of a distinct form. I formerly assigned reasons for rejecting the belief that the mother-plant is affected through the intervention of the hybridised embryo; but even if this view were admitted, the case would become one of graft-hybridism, for the fertilised embryo and the mother-plant must be looked at as distinct individuals.

With animals which do not breed until nearly mature, and of which all the parts are then fully developed, it is hardly possible that the male element should directly affect the female. But we have the analogous and perfectly well-ascertained case of the male element of a distinct form, as with the quagga and Lord Morton's mare, affecting the ovarium of the female, so that the ovules and offspring subsequently produced by her when impregnated by other males are plainly affected and hybridised by the first male.

Development.—The fertilised germ reaches maturity by a vast number of changes: these are either slight and slowly effected, as when the child grows into the man, or are great and sudden, as with the metamorphoses of most insects. Between these extremes we have, even within the same class, every gradation: thus, as Sir J. Lubbock has shown, there is an Ephemerous insect which moults above twenty times, undergoing each time a slight but decided change of structure; and these changes, as he further remarks, probably reveal to us the normal stages of development which are concealed and hurried through, or suppressed, in most other insects. In ordinary metamorphoses, the parts and organs appear to become changed into the corresponding parts in the next stage of development; but there is another form of development, which has been called by Professor Owen metagenesis. In this case "the new parts are not moulded upon the inner surface of the old ones. The plastic force has changed its course of operation. The outer case, and all that gave form and character to the precedent individual, perish and are cast off; they are not changed into the corresponding parts of the new individual. These are due to a new and distinct developmental process," &c. Metamorphosis, however, graduates so insensibly into metagenesis, that the two processes cannot be distinctly separated. For instance, in the last change which Cirripedes undergo, the alimentary canal and some other organs are moulded on pre-existing parts; but the eyes of the old and the young animal are developed in entirely different parts of the body; the tips of the mature limbs are formed within the larval limbs, and may be said to be metamorphosed from them; but their basal portions and the whole thorax are developed in a plane actually at right angles to the limbs and thorax of the larva; and this may be called metagenesis. The metagenetic process is carried to an extreme degree in the development of some Echinoderms, for the animal in the second stage of development is formed almost like a bud within the animal of the first stage, the latter being then cast off like an old vestment, yet sometimes still maintaining for a short period an independent vitality.

If, instead of a single individual, several were to be thus developed metagenetically within a pre-existing form, the process would be called one of alternate generation. The young thus developed may either closely resemble the encasing parent-form, as with the larvæ of Cecidomyia, or may differ to an astonishing degree, as with many parasitic worms and with jelly-fishes; but this does not make any essential difference in the process, any more than the greatness or abruptness of the change in the metamorphoses of insects.

The whole question of development is of great importance for our present subject. When an organ, the eye for instance, is metagenetically formed in a part of the body where during the previous stage of development no eye existed, we must look at it as a new and independent growth. The absolute independence of new and old structures, which correspond in structure and function, is still more obvious when several individuals are formed within a previous encasing form, as in the cases of alternate generation. The same important principle probably comes largely into play even in the case of continuous growth, as we shall see when we consider the inheritance of modifications at corresponding ages.

We are led to the same conclusion, namely, the independence of parts successively developed, by another and quite distinct group of facts. It is well known that many animals belonging to the same class, and therefore not differing widely from each other, pass through an extremely different course of development. Thus certain beetles, not in any way remarkably different from others of the same order, undergo what has been called a hyper-metamorphosis—that is, they pass through an early stage wholly different from the ordinary grub-like larva. In the same sub-order of crabs, namely, the Macroura, as Fritz Müller remarks, the river cray-fish is hatched under the same form which it ever afterwards retains; the young lobster has divided legs, like a Mysis; the Palæmon appears under the form of a Zoea, and Peneus under the Nauplius-form; and how wonderfully these larval forms differ from each other, is known to every naturalist. Some other crustaceans, as the same author observes, start from the same point and arrive at nearly the same end, but in the middle of their development are widely different from each other. Still more striking cases could be given with respect to the Echinodermata. With the Medusæ or jelly-fishes Professor Allman observes, "the classification of the Hydroida would be a comparatively simple task if, as has been erroneously asserted, generically-identical medusoids always arose from generically-identical polypoids; and on the other hand, that generically-identical polypoids always gave origin to generically-identical medusoids." So, again, Dr. Strethill Wright remarks, "in the life-history of the Hydroidæ any phase, planuloid, polypoid, or medusoid, may be absent."

According to the belief now generally accepted by our best naturalists, all the members of the same order or class, the Macrourous crustaceans for instance, are descended from a common progenitor. During their descent they have diverged much in structure, but have retained much in common; and this divergence and retention of character has been effected, though they have passed and still pass through marvellously different metamorphoses. This fact well illustrates how independent each structure must be from that which precedes and follows it in the course of development.

The Functional Independence of the Elements or Units of the Body.—Physiologists agree that the whole organism consists of a multitude of elemental parts, which are to a great extent independent of each other. Each organ, says Claude Bernard, has its proper life, its autonomy; it can develop and reproduce itself independently of the adjoining tissues. The great German authority, Virchow, asserts still more emphatically that each system, as the nervous or osseous system, or the blood, consists of an "enormous mass of minute centres of action.... Every element has its own special action, and even though it derive its stimulus to activity from other parts, yet alone effects the actual performance of its duties.... Every single epithelial and muscular fibre-cell leads a sort of parasitical existence in relation to the rest of the body.... Every single bone-corpuscle really possesses conditions of nutrition peculiar to itself." Each element, as Mr. Paget remarks, lives its appointed time, and then dies, and, after being cast off or absorbed, is replaced. I presume that no physiologist doubts that, for instance, each bone-corpuscle of the finger differs from the corresponding corpuscle in the corresponding joint of the toe; and there can hardly be a doubt that even those on the corresponding sides of the body differ, though almost identical in nature. This near approach to identity is curiously shown in many diseases in which the same exact points on the right and left sides of the body are similarly affected; thus Mr. Paget gives a drawing of a diseased pelvis, in which the bone has grown into a most complicated pattern, but "there is not one spot or line on one side which is not represented, as exactly as it would be in a mirror, on the other."

Many facts support this view of the independent life of each minute element of the body. Virchow insists that a single bone-corpuscle or a single cell in the skin may become diseased. The spur of a cock, after being inserted into the eye of an ox, lived for eight years, and acquired a weight of 306 grammes, or nearly fourteen ounces. The tail of a pig has been grafted into the middle of its back, and reacquired sensibility. Dr. Ollier inserted a piece of periosteum from the bone of a young dog under the skin of a rabbit, and true bone was developed. A multitude of similar facts could be given. The frequent presence of hairs and of perfectly developed teeth, even teeth of the second dentition, in ovarian tumours, are facts leading to the same conclusion.

Whether each of the innumerable autonomous elements of the body is a cell or the modified product of a cell, is a more doubtful question, even if so wide a definition be given to the term, as to include cell-like bodies without walls and without nuclei. Professor Lionel Beale uses the term "germinal matter" for the contents of cells, taken in this wide acceptation, and he draws a broad distinction between germinal matter and "formed material" or the various products of cells. But the doctrine of omnis cellula e cellulâ is admitted for plants, and is a widely prevalent belief with respect to animals. Thus Virchow, the great supporter of the cellular theory, whilst allowing that difficulties exist, maintains that every atom of tissue is derived from cells, and these from pre-existing cells, and these primarily from the egg, which he regards as a great cell. That cells, still retaining the same nature, increase by self-division or proliferation, is admitted by almost every one. But when an organism undergoes a great change of structure during development, the cells, which at each stage are supposed to be directly derived from previously-existing cells, must likewise be greatly changed in nature; this change is apparently attributed by the supporters of the cellular doctrine to some inherent power which the cells possess, and not to any external agency.

Another school maintains that cells and tissues of all kinds may be formed, independently of pre-existing cells, from plastic lymph or blastema; and this it is thought is well exhibited in the repair of wounds. As I have not especially attended to histology, it would be presumptuous in me to express an opinion on the two opposed doctrines. But every one appears to admit that the body consists of a multitude of "organic units," each of which possesses its own proper attributes, and is to a certain extent independent of all others. Hence it will be convenient to use indifferently the terms cells or organic units or simply units.

Variability and Inheritance.—We have seen in the twenty-second chapter that variability is not a principle co-ordinate with life or reproduction, but results from special causes, generally from changed conditions acting during successive generations. Part of the fluctuating variability thus induced is apparently due to the sexual system being easily affected by changed conditions, so that it is often rendered impotent; and when not so seriously affected, it often fails in its proper function of transmitting truly the characters of the parents to the offspring. But variability is not necessarily connected with the sexual system, as we see from the cases of bud-variation; and although we may not be able to trace the nature of the connexion, it is probable that many deviations of structure which appear in sexual offspring result from changed conditions acting directly on the organisation, independently of the reproductive organs. In some instances we may feel sure of this, when all, or nearly all the individuals which have been similarly exposed are similarly and definitely affected—as in the dwarfed and otherwise changed maize brought from hot countries when cultivated in Germany; in the change of the fleece in sheep within the tropics; to a certain extent in the increased size and early maturity of our highly-improved domesticated animals; in inherited gout from intemperance; and in many other such cases. Now, as such changed conditions do not especially affect the reproductive organs, it seems mysterious on any ordinary view why their product, the new organic being, should be similarly affected.

How, again, can we explain to ourselves the inherited effects of the use or disuse of particular organs? The domesticated duck flies less and walks more than the wild duck, and its limb-bones have become in a corresponding manner diminished and increased in comparison with those of the wild duck. A horse is trained to certain paces, and the colt inherits similar consensual movements. The domesticated rabbit becomes tame from close confinement; the dog intelligent from associating with man; the retriever is taught to fetch and carry: and these mental endowments and bodily powers are all inherited. Nothing in the whole circuit of physiology is more wonderful. How can the use or disuse of a particular limb or of the brain affect a small aggregate of reproductive cells, seated in a distant part of the body, in such a manner that the being developed from these cells inherits the characters of either one or both parents? Even an imperfect answer to this question would be satisfactory.

Sexual reproduction does not essentially differ, as we have seen, from budding or self-division, and these processes graduate through the repair of injuries into ordinary development and growth; it might therefore be expected that every character would be as regularly transmitted by all the methods of reproduction as by continued growth. In the chapters devoted to inheritance it was shown that a multitude of newly-acquired characters, whether injurious or beneficial, whether of the lowest or highest vital importance, are often faithfully transmitted—frequently even when one parent alone possesses some new peculiarity. It deserves especial attention that characters appearing at any age tend to reappear at a corresponding age. We may on the whole conclude that in all cases inheritance is the rule, and non-inheritance the anomaly. In some instances a character is not inherited, from the conditions of life being directly opposed to its development; in many instances, from the conditions incessantly inducing fresh variability, as with grafted fruit-trees and highly cultivated flowers. In the remaining cases the failure may be attributed to reversion, by which the child resembles its grandparents or more remote progenitors, instead of its parents.

This principle of Reversion is the most wonderful of all the attributes of Inheritance. It proves to us that the transmission of a character and its development, which ordinarily go together and thus escape discrimination, are distinct powers; and these powers in some cases are even antagonistic, for each acts alternately in successive generations. Reversion is not a rare event, depending on some unusual or favourable combination of circumstances, but occurs so regularly with crossed animals and plants, and so frequently with uncrossed breeds, that it is evidently an essential part of the principle of inheritance. We know that changed conditions have the power of evoking long-lost characters, as in the case of some feral animals. The act of crossing in itself possesses this power in a high degree. What can be more wonderful than that characters, which have disappeared during scores, or hundreds, or even thousands of generations, should suddenly reappear perfectly developed, as in the case of pigeons and fowls when purely bred, and especially when crossed; or as with the zebrine stripes on dun-coloured horses, and other such cases? Many monstrosities come under this same head, as when rudimentary organs are redeveloped, or when an organ which we must believe was possessed by an early progenitor, but of which not even a rudiment is left, suddenly reappears, as with the fifth stamen in some Scrophulariaceæ. We have already seen that reversion acts in bud-reproduction; and we know that it occasionally acts during the growth of the same individual animal, especially, but not exclusively, when of crossed parentage,—as in the rare cases described of individual fowls, pigeons, cattle, and rabbits, which have reverted as they advanced in years to the colours of one of their parents or ancestors.

We are led to believe, as formerly explained, that every character which occasionally reappears is present in a latent form in each generation, in nearly the same manner as in male and female animals secondary characters of the opposite sex lie latent, ready to be evolved when the reproductive organs are injured. This comparison of the secondary sexual characters which are latent in both sexes, with other latent characters, is the more appropriate from the case recorded of the Hen, which assumed some of the masculine characters, not of her own race, but of an early progenitor; she thus exhibited at the same time the redevelopment of latent characters of both kinds and connected both classes. In every living creature we may feel assured that a host of lost characters lie ready to be evolved under proper conditions. How can we make intelligible, and connect with other facts, this wonderful and common capacity of reversion,—this power of calling back to life long-lost characters?

I have now enumerated the chief facts which every one would desire to connect by some intelligible bond. This can be done, as it seems to me, if we make the following assumptions; if the first and chief one be not rejected, the others, from being supported by various physiological considerations, will not appear very improbable. It is almost universally admitted that cells, or the units of the body, propagate themselves by self-division or proliferation, retaining the same nature, and ultimately becoming converted into the various tissues and substances of the body. But besides this means of increase I assume that cells, before their conversion into completely passive or "formed material," throw off minute granules or atoms, which circulate freely throughout the system, and when supplied with proper nutriment multiply by self-division, subsequently becoming developed into cells like those from which they were derived. These granules for the sake of distinctness may be called cell-gemmules, or, as the cellular theory is not fully established, simply gemmules. They are supposed to be transmitted from the parents to the offspring, and are generally developed in the generation which immediately succeeds, but are often transmitted in a dormant state during many generations and are then developed. Their development is supposed to depend on their union with other partially developed cells or gemmules which precede them in the regular course of growth. Why I use the term union, will be seen when we discuss the direct action of pollen on the tissues of the mother-plant. Gemmules are supposed to be thrown off by every cell or unit, not only during the adult state, but during all the stages of development. Lastly, I assume that the gemmules in their dormant state have a mutual affinity for each other, leading to their aggregation either into buds or into the sexual elements. Hence, speaking strictly, it is not the reproductive elements, nor the buds, which generate new organisms, but the cells themselves throughout the body. These assumptions constitute the provisional hypothesis which I have called Pangenesis. Nearly similar views have been propounded, as I find, by other authors, more especially by Mr. Herbert Spencer; but they are here modified and amplified.

Before proceeding to show, firstly, how far these assumptions are in themselves probable, and secondly, how far they connect and explain the various groups of facts with which we are concerned, it may be useful to give an illustration of the hypothesis. If one of the simplest Protozoa be formed, as appears under the microscope, of a small mass of homogeneous gelatinous matter, a minute atom thrown off from any part and nourished under favourable circumstances would naturally reproduce the whole; but if the upper and lower surfaces were to differ in texture from the central portion, then all three parts would have to throw off atoms or gemmules, which when aggregated by mutual affinity would form either buds or the sexual elements. Precisely the same view may be extended to one of the higher animals; although in this case many thousand gemmules must be thrown off from the various parts of the body. Now, when the leg, for instance, of a salamander is cut off, a slight crust forms over the wound, and beneath this crust the uninjured cells or units of bone, muscle, nerves, &c., are supposed to unite with the diffused gemmules of those cells which in the perfect leg come next in order; and these as they become slightly developed unite with others, and so on until a papilla of soft cellular tissue, the "budding leg," is formed, and in time a perfect leg. Thus, that portion of the leg which had been cut off, neither more nor less, would be reproduced. If the tail or leg of a young animal had been cut off, a young tail or leg would have been reproduced, as actually occurs with the amputated tail of the tadpole; for gemmules of all the units which compose the tail are diffused throughout the body at all ages. But during the adult state the gemmules of the larval tail would remain dormant, for they would not meet with pre-existing cells in a proper state of development with which to unite. If from changed conditions or any other cause any part of the body should become permanently modified, the gemmules, which are merely minute portions of the contents of the cells forming the part, would naturally reproduce the same modification. But gemmules previously derived from the same part before it had undergone any change, would still be diffused throughout the organisation, and would be transmitted from generation to generation, so that under favourable circumstances they might be redeveloped, and then the new modification would be for a time or for ever lost. The aggregation of gemmules derived from every part of the body, through their mutual affinity, would form buds, and their aggregation in some special manner, apparently in small quantity, together probably with the presence of gemmules of certain primordial cells, would constitute the sexual elements. By means of these illustrations the hypothesis of pangenesis has, I hope, been rendered intelligible.

Physiologists maintain, as we have seen, that each cell, though to a large extent dependent on others, is likewise, to a certain extent, independent or autonomous. I go one small step further, and assume that each cell casts off a free gemmule, which is capable of reproducing a similar cell. There is some analogy between this view and what we see in compound animals and in the flower-buds on the same tree; for these are distinct individuals capable of true or seminal reproduction, yet have parts in common and are dependent on each other; thus the tree has its bark and trunk, and certain corals, as the Virgularia, have not only parts, but movements in common.

The existence of free gemmules is a gratuitous assumption, yet can hardly be considered as very improbable, seeing that cells have the power of multiplication through the self-division of their contents. Gemmules differ from true ovules or buds inasmuch as they are supposed to be capable of multiplication in their undeveloped state. No one probably will object to this capacity as improbable. The blastema within the egg has been known to divide and give birth to two embryos; and Thuret has seen the zoospore of an alga divide itself, and both halves germinate. An atom of small-pox matter, so minute as to be borne by the wind, must multiply itself many thousand-fold in a person thus inoculated. It has recently been ascertained that a minute portion of the mucous discharge from an animal affected with rinderpest, if placed in the blood of a healthy ox, increases so fast that in a short space of time "the whole mass of blood, weighing many pounds, is infected, and every small particle of that blood contains enough poison to give, within less than forty-eight hours, the disease to another animal."

The retention of free and undeveloped gemmules in the same body from early youth to old age may appear improbable, but we should remember how long seeds lie dormant in the earth and buds in the bark of a tree. Their transmission from generation to generation may appear still more improbable; but here again we should remember that many rudimentary and useless organs are transmitted and have been transmitted during an indefinite number of generations. We shall presently see how well the long-continued transmission of undeveloped gemmules explains many facts.

As each unit, or group of similar units throughout the body, casts off its gemmules, and as all are contained within the smallest egg or seed, and within each spermatozoon or pollen-grain, their number and minuteness must be something inconceivable. I shall hereafter recur to this objection, which at first appears so formidable; but it may here be remarked that a cod-fish has been found to produce 4,872,000 eggs, a single Ascaris about 64,000,000 eggs, and a single Orchidaceous plant probably as many million seeds. In these several cases, the spermatozoa and pollen-grains must exist in considerably larger numbers. Now, when we have to deal with numbers such as these, which the human intellect cannot grasp, there is no good reason for rejecting our present hypothesis on account of the assumed existence of cell-gemmules a few thousand times more numerous.

The gemmules in each organism must be thoroughly diffused; nor does this seem improbable considering their minuteness, and the steady circulation of fluids throughout the body. So it must be with the gemmules of plants, for with certain kinds even a minute fragment of a leaf will reproduce the whole. But a difficulty here occurs; it would appear that with plants, and probably with compound animals, such as corals, the gemmules do not spread from bud to bud, but only through the tissues developed from each separate bud. We are led to this conclusion from the stock being rarely affected by the insertion of a bud or graft from a distinct variety. This non-diffusion of the gemmules is still more plainly shown in the case of ferns; for Mr. Bridgman has proved that, when spores (which it should be remembered are of the nature of buds) are taken from a monstrous part of a frond, and others from an ordinary part, each reproduces the form of the part whence derived. But this non-diffusion of the gemmules from bud to bud may be only apparent, depending, as we shall hereafter see, on the nature of the first-formed cells in the buds.

The assumed elective affinity of each gemmule for that particular cell which precedes it in the order of development is supported by many analogies. In all ordinary cases of sexual reproduction the male and female elements have a mutual affinity for each other: thus, it is believed that about ten thousand species of Compositæ exist, and there can be no doubt that if the pollen of all these species could be, simultaneously or successively, placed on the stigma of any one species, this one would elect with unerring certainty its own pollen. This elective capacity is all the more wonderful, as it must have been acquired since the many species of this great group of plants branched off from a common progenitor. On any view of the nature of sexual reproduction, the protoplasm contained within the ovules and within the sperm-cells (or the "spermatic force" of the latter, if so vague a term be preferred) must act on each other by some law of special affinity, either during or subsequently to impregnation, so that corresponding parts alone affect each other; thus, a calf produced from a short-horned cow by a long-horned bull has its horns and not its horny hoofs affected by the union of the two forms, and the offspring from two birds with differently coloured tails have their tails and not their whole plumage affected.

The various tissues of the body plainly show, as many physiologists have insisted, an affinity for special organic substances, whether natural or foreign to the body. We see this in the cells of the kidneys attracting urea from the blood; in the worrara poison affecting the nerves; upas and digitalis the muscles; the Lytta vesicatoria the kidneys; and in the poisonous matter of many diseases, as small-pox, scarlet-fever, hooping-cough, glanders, cancer, and hydrophobia, affecting certain definite parts of the body or certain tissues or glands.

The affinity of various parts of the body for each other during their early development was shown in the last chapter, when discussing the tendency to fusion in homologous parts. This affinity displays itself in the normal fusion of organs which are separate at an early embryonic age, and still more plainly in those marvellous cases of double monsters in which each bone, muscle, vessel, and nerve in the one embryo, blends with the corresponding part in the other. The affinity between homologous organs may come into action with single parts, or with the entire individual, as in the case of flowers or fruits which are symmetrically blended together with all their parts doubled, but without any other trace of fusion.

It has also been assumed that the development of each gemmule depends on its union with another cell or unit which has just commenced its development, and which, from preceding it in order of growth, is of a somewhat different nature. Nor is it a very improbable assumption that the development of a gemmule is determined by its union with a cell slightly different in nature, for abundant evidence was given in the seventeenth chapter, showing that a slight degree of differentiation in the male and female sexual elements favours in a marked manner their union and subsequent development. But what determines the development of the gemmules of the first-formed or primordial cell in the unimpregnated ovule, is beyond conjecture.

It must also be admitted that analogy fails to guide us towards any determination on several other points: for instance, whether cells, derived from the same parent-cell, may, in the regular course of growth, become developed into different structures, from absorbing peculiar kinds of nutriment, independently of their union with distinct gemmules. We shall appreciate this difficulty if we call to mind, what complex yet symmetrical growths the cells of plants yield when they are inoculated by the poison of a gall-insect. With animals various polypoid excrescences and tumours are now generally admitted to be the direct product, through proliferation, of normal cells which have become abnormal. In the regular growth and repair of bones, the tissues undergo, as Virchow remarks, a whole series of permutations and substitutions. "The cartilage-cells may be converted by a direct transformation into marrow-cells, and continue as such; or they may first be converted into osseous and then into medullary tissue; or lastly, they may first be converted into marrow and then into bone. So variable are the permutations of these tissues, in themselves so nearly allied, and yet in their external appearance so completely distinct." But as these tissues thus change their nature at any age, without any obvious change in their nutrition, we must suppose in accordance with our hypothesis that gemmules derived from one kind of tissue combine with the cells of another kind, and cause the successive modifications.

It is useless to speculate at what period of development each organic unit casts off its gemmules; for the whole subject of the development of the various elemental tissues is as yet involved in much doubt. Some physiologists, for instance, maintain that muscle or nerve-fibres are developed from cells, which are afterwards nourished by their own proper powers of absorption; whilst other physiologists deny their cellular origin; and Beale maintains that such fibres are renovated exclusively by the conversion of fresh germinal matter (that is the so-called nuclei) into "formed material." However this may be, it appears probable that all external agencies, such as changed nutrition, increased use or disuse, &c., which induced any permanent modification in a structure, would at the same time or previously act on the cells, nuclei, germinal or formative matter, from which the structures in question were developed, and consequently would act on the gemmules or cast-off atoms.

There is another point on which it is useless to speculate, namely, whether all gemmules are free and separate, or whether some are from the first united into small aggregates. A feather, for instance, is a complex structure, and, as each separate part is liable to inherited variations, I conclude that each feather certainly generates a large number of gemmules; but it is possible that these may be aggregated into a compound gemmule. The same remark applies to the petals of a flower, which in some cases are highly complex, with each ridge and hollow contrived for special purposes, so that each part must have been separately modified, and the modifications transmitted; consequently, separate gemmules, according to our hypothesis, must have been thrown off from each cell or part. But, as we sometimes see half an anther or a small portion of a filament becoming petaliform, or parts or mere stripes of the calyx assuming the colour and texture of the corolla, it is probable that with petals the gemmules of each cell are not aggregated together into a compound gemmule, but are freely and separately diffused.

Having now endeavoured to show that the several foregoing assumptions are to a certain extent supported by analogous facts, and having discussed some of the most doubtful points, we will consider how far the hypothesis brings under a single point of view the various cases enumerated in the First Part. All the forms of reproduction graduate into each other and agree in their product; for it is impossible to distinguish between organisms produced from buds, from self-division, or from fertilised germs; such organisms are liable to variations of the same nature and to reversion of character; and as we now see that all the forms of reproduction depend on the aggregation of gemmules derived from the whole body, we can understand this general agreement. It is satisfactory to find that sexual and asexual generation, by both of which widely different processes the same living creature is habitually produced, are fundamentally the same. Parthenogenesis is no longer wonderful; in fact, the wonder is that it should not oftener occur. We see that the reproductive organs do not actually create the sexual elements; they merely determine or permit the aggregation of the gemmules in a special manner. These organs, together with their accessory parts, have, however, high functions to perform; they give to both elements a special affinity for each other, independently of the contents of the male and female cells, as is shown in the case of plants by the mutual reaction of the stigma and pollen-grains; they adapt one or both elements for independent temporary existence, and for mutual union. The contrivances for these purposes are sometimes wonderfully complex, as with the spermatophores of the Cephalopoda. The male element sometimes possesses attributes which, if observed in an independent animal, would be put down to instinct guided by sense-organs, as when the spermatozoon of an insect finds its way into the minute micropyle of the egg, or as when the antherozoids of certain algæ swim by the aid of their ciliæ to the female plant, and force themselves into a minute orifice. In these latter cases, however, we must believe that the male element has acquired its powers, on the same principle with the larvæ of animals, namely by successive modifications developed at corresponding periods of life: we can hardly avoid in these cases looking at the male element as a sort of premature larva, which unites, or, like one of the lower algæ, conjugates, with the female element. What determines the aggregation of the gemmules within the sexual organs we do not in the least know; nor do we know why buds are formed in certain definite places, leading to the symmetrical growth of trees and corals, nor why adventitious buds may be formed almost anywhere, even on a petal, and frequently upon healed wounds. As soon as the gemmules have aggregated themselves, development apparently commences, but in the case of buds is often afterwards suspended, and in the case of the sexual elements soon ceases, unless the elements of the opposite sexes combine; even after this has occurred, the fertilised germ, as with seeds buried in the ground, may remain during a lengthened period in a dormant state.

The antagonism which has long been observed, though exceptions occur, between active growth and the power of sexual reproduction—between the repair of injuries and gemmation—and with plants, between rapid increase by buds, rhizomes, &c., and the production of seed, is partly explained by the gemmules not existing in sufficient numbers for both processes. But this explanation hardly applies to those plants which naturally produce a multitude of seeds, but which, through a comparatively small increase in the number of the buds on their rhizomes or offsets, yield few or no seed. As, however, we shall presently see that buds probably include tissue which has already been to a certain extent developed or differentiated, some additional organised matter will thus have been expended.

From one of the forms of Reproduction, namely, spontaneous self-division, we are led by insensible steps to the repair of the slightest injury; and the existence of gemmules, derived from every cell or unit throughout the body and everywhere diffused, explains all such cases,—even the wonderful fact that, when the limbs of the salamander were cut off many times successively by Spallanzani and Bonnet, they were exactly and completely reproduced. I have heard this process compared with the recrystallisation which occurs when the angles of a broken crystal are repaired; and the two processes have this much in common, that in the one case the polarity of the molecules is the efficient cause, and in the other the affinity of the gemmules for particular nascent cells.

Pangenesis does not throw much light on Hybridism, but agrees well with most of the ascertained facts. We may conclude from the fact of a single spermatozoon or pollen-grain being insufficient for impregnation, that a certain number of gemmules derived from each cell or unit are required for the development of each part. From the occurrence of parthenogenesis, more especially in the case of the silk-moth, in which the embryo is often partially formed, we may also infer that the female element includes nearly sufficient gemmules of all kinds for independent development, so that when united with the male element the gemmules must be superabundant. Now, as a general rule, when two species or races are crossed reciprocally, the offspring do not differ, and this shows that both sexual elements agree in power, in accordance with the view that they include the same gemmules. Hybrids and mongrels are generally intermediate in character between the two parent-forms, yet occasionally they closely resemble one parent in one part and the other parent in another part, or even in their whole structure: nor is this difficult to understand on the admission that the gemmules in the fertilised germ are superabundant in number, and that those derived from one parent have some advantage in number, affinity, or vigour over those derived from the other parent. Crossed forms sometimes exhibit the colour or other characters of either parent in stripes or blotches; and this may occur in the first generation, or through reversion in succeeding bud and seminal generations, as in the several instances given in the eleventh chapter. In these cases we must follow Naudin, and admit that the "essence" or "element" of the two species, which terms I should translate into the gemmules, have an affinity for their own kind, and thus separate themselves into distinct stripes or blotches; and reasons were given, when discussing in the fifteenth chapter the incompatibility of certain characters to unite, for believing in such mutual affinity. When two forms are crossed, one is not rarely found to be prepotent in the transmission of character over the other; and this we can explain only by again assuming that the one form has some advantage in the number, vigour, or affinity of its gemmules, except in those cases, where certain characters are present in the one form and latent in the other. For instance, there is a latent tendency in all pigeons to become blue, and, when a blue pigeon is crossed with one of any other colour, the blue tint is generally prepotent. When we consider latent characters, the explanation of this form of prepotency will be obvious.

When one species is crossed with another it is notorious that they do not yield the full or proper number of offspring; and we can only say on this head that, as the development of each organism depends on such nicely-balanced affinities between a host of gemmules and developing cells or units, we need not feel at all surprised that the commixture of gemmules derived from two distinct species should lead to a partial or complete failure of development. With respect to the sterility of hybrids produced from the union of two distinct species, it was shown in the nineteenth chapter that this depends exclusively on the reproductive organs being specially affected; but why these organs should be thus affected we do not know, any more than why unnatural conditions of life, though compatible with health, should cause sterility; or why continued close interbreeding, or the illegitimate unions of dimorphic and trimorphic plants, induce the same result. The conclusion that the reproductive organs alone are affected, and not the whole organisation, agrees perfectly with the unimpaired or even increased capacity in hybrid plants for propagation by buds; for this implies, according to our hypothesis, that the cells of the hybrids throw off hybridised cell-gemmules, which become aggregated into buds, but fail to become aggregated within the reproductive organs, so as to form the sexual elements. In a similar manner many plants, when placed under unnatural conditions, fail to produce seed, but can readily be propagated by buds. We shall presently see that pangenesis agrees well with the strong tendency to reversion exhibited by all crossed animals and plants.

It was shown in the discussion on graft-hybrids that there is some reason to believe that portions of cellular tissue taken from distinct plants become so intimately united, as afterwards occasionally to produce crossed or hybridised buds. If this fact were fully established, it would, by the aid of our hypothesis, connect gemmation and sexual reproduction in the closest manner.

Abundant evidence has been advanced proving that pollen taken from one species or variety and applied to the stigma of another sometimes directly affects the tissues of the mother-plant. It is probable that this occurs with many plants during fertilisation, but can only be detected when distinct forms are crossed. On any ordinary theory of reproduction this is a most anomalous circumstance, for the pollen-grains are manifestly adapted to act on the ovule, but in these cases they act on the colour, texture, and form of the coats of the seeds, on the ovarium itself, which is a modified leaf, and even on the calyx and upper part of the flower-peduncle. In accordance with the hypothesis of pangenesis pollen includes gemmules, derived from every part of the organisation, which diffuse themselves and multiply by self-division; hence it is not surprising that gemmules within the pollen, which are derived from the parts near the reproductive organs, should sometimes be able to affect the same parts, whilst still undergoing development, in the mother-plant.

As, during all the stages of development, the tissues of plants consist of cells, and as new cells are not known to be formed between, or independently of, pre-existing cells, we must conclude that the gemmules derived from the foreign pollen do not become developed merely in contact with pre-existing cells, but actually penetrate the nascent cells of the mother-plant. This process may be compared with the ordinary act of fertilisation, during which the contents of the pollen-tubes penetrate the closed embryonic sack within the ovule, and determine the development of the embryo. According to this view, the cells of the mother-plant may almost literally be said to be fertilised by the gemmules derived from the foreign pollen. With all organisms, as we shall presently see, the cells or organic units of the embryo during the successive stages of development may in like manner be said to be fertilised by the gemmules of the cells, which come next in the order of formation.

Animals, when capable of sexual reproduction, are fully developed, and it is scarcely possible that the male element should affect the tissues of the mother in the same direct manner as with plants; nevertheless it is certain that her ovaria are sometimes affected by a previous impregnation, so that the ovules subsequently fertilised by a distinct male are plainly influenced in character; and this, as in the case of foreign pollen, is intelligible through the diffusion, retention, and action of the gemmules included within the spermatozoa of the previous male.

Each organism reaches maturity through a longer or shorter course of development. The changes may be small and insensibly slow, as when a child grows into a man, or many, abrupt, and slight, as in the metamorphoses of certain ephemerous insects, or again few and strongly marked, as with most other insects. Each part may be moulded within a previously existing and corresponding part, and in this case it will appear, falsely as I believe, to be formed from the old part; or it may be developed within a wholly distinct part of the body, as in the extreme cases of metagenesis. An eye, for instance, may be developed at a spot where no eye previously existed. We have also seen that allied organic beings in the course of their metamorphoses sometimes attain nearly the same structure after passing through widely different forms; or conversely, after passing through nearly the same early forms, arrive at a widely different termination. In these cases it is very difficult to believe that the early cells or units possess the inherent power, independently of any external agent, of producing new structures wholly different in form, position, and function. But these cases become plain on the hypothesis of pangenesis. The organic units, during each stage of development, throw off gemmules, which, multiplying, are transmitted to the offspring. In the offspring, as soon as any particular cell or unit in the proper order of development becomes partially developed, it unites with (or to speak metaphorically is fertilised by) the gemmule of the next succeeding cell, and so onwards. Now, supposing that at any stage of development, certain cells or aggregates of cells had been slightly modified by the action of some disturbing cause, the cast-off gemmules or atoms of the cell-contents could hardly fail to be similarly affected, and consequently would reproduce the same modification. This process might be repeated until the structure of the part at this particular stage of development became greatly changed, but this would not necessarily affect other parts whether previously or subsequently developed. In this manner we can understand the remarkable independence of structure in the successive metamorphoses, and especially in the successive metageneses of many animals.

The term growth ought strictly to be confined to mere increase of size, and development to change of structure. Now, a child is said to grow into a man, and a foal into a horse, but, as in these cases there is much change of structure, the process properly belongs to the order of development. We have indirect evidence of this in many variations and diseases supervening during so-called growth at a particular period, and being inherited at a corresponding period. In the case, however, of diseases which supervene during old age, subsequently to the ordinary period of procreation, and which nevertheless are sometimes inherited, as occurs with brain and heart complaints, we must suppose that the organs were in fact affected at an earlier age and threw off at this period affected gemmules; but that the affection became visible or injurious only after the prolonged growth of the part in the strict sense of the word. In all the changes of structure which regularly supervene during old age, we see the effects of deteriorated growth, and not of true development.

In the so-called process of alternate generation many individuals are generated asexually during very early or later stages of development. These individuals may closely resemble the preceding larval form, but generally are wonderfully dissimilar. To understand this process we must suppose that at a certain stage of development the gemmules are multiplied at an unusual rate, and become aggregated by mutual affinity at many centres of attraction, or buds. These buds, it may be remarked, must include gemmules not only of all the succeeding but likewise of all the preceding stages of development; for when mature they have the power of transmitting by sexual generation gemmules of all the stages, however numerous these may be. It was shown in the First Part, at least in regard to animals, that the new beings which are thus at any period asexually generated do not retrograde in development—that is, they do not pass through those earlier stages, through which the fertilised germ of the same animal has to pass; and an explanation of this fact was attempted as far as the final or teleological cause is concerned. We can likewise understand the proximate cause, if we assume, and the assumption is far from improbable, that buds, like chopped-up fragments of a hydra, are formed of tissue which has already passed through several of the earlier stages of development; for in this case their component cells or units would not unite with the gemmules derived from the earlier-formed cells, but only with those which came next in the order of development. On the other hand, we must believe that, in the sexual elements, or probably in the female alone, gemmules of certain primordial cells are present; and these, as soon as their development commences, unite in due succession with the gemmules of every part of the body, from the first to the last period of life.

The principle of the independent formation of each part, in so far as its development depends on the union of the proper gemmules with certain nascent cells, together with the superabundance of the gemmules derived from both parents and self-multiplied, throws light on a widely different group of facts, which on any ordinary view of development appears very strange. I allude to organs which are abnormally multiplied or transposed. Thus gold-fish often have supernumerary fins placed on various parts of their bodies. We have seen that, when the tail of a lizard is broken off, a double tail is sometimes reproduced, and when the foot of the salamander is divided longitudinally, additional digits are occasionally formed. When frogs, toads, &c., are born with their limbs doubled, as sometimes occurs, the doubling, as Gervais remarks, cannot be due to the complete fusion of two embryos, with the exception of the limbs, for the larvæ are limbless. The same argument is applicable to certain insects produced with multiple legs or antennæ, for these are metamorphosed from apodal or antennæless larvæ. Alphonse Milne-Edwards has described the curious case of a crustacean in which one eye-peduncle supported, instead of a complete eye, only an imperfect cornea, out of the centre of which a portion of an antenna was developed. A case has been recorded of a man who had during both dentitions a double tooth in place of the left second incisor, and he inherited this peculiarity from his paternal grandfather. Several cases are known of additional teeth having been developed in the palate, more especially with horses, and in the orbit of the eye. Certain breeds of sheep bear a whole crowd of horns on their foreheads. Hairs occasionally appear in strange situations, as within the ears of the Siamese hairy family; and hairs "quite natural in structure" have been observed "within the substance of the brain." As many as five spurs have been seen on both legs in certain Game-fowls. In the Polish fowl the male is ornamented with a topknot of hackles like those on his neck, whilst the female has one of common feathers. In feather-footed pigeons and fowls, feathers like those on the wing arise from the outer side of the legs and toes. Even the elemental parts of the same feather may be transposed; for in the Sebastopol goose, barbules are developed on the divided filaments of the shaft.

Analogous cases are of such frequent occurrence with plants that they do not strike us with sufficient surprise. Supernumerary petals, stamens, and pistils, are often produced. I have seen a leaflet low down in the compound leaf of Vicia sativa converted into a tendril, and a tendril possesses many peculiar properties, such as spontaneous movement and irritability. The calyx sometimes assumes, either wholly or by stripes, the colour and texture of the corolla. Stamens are so frequently converted, more or less completely, into petals, that such cases are passed over as not deserving notice; but as petals have special functions to perform, namely, to protect the included organs, to attract insects, and in not a few cases to guide their entrance by well-adapted contrivances, we can hardly account for the conversion of stamens into petals merely by unnatural or superfluous nourishment. Again, the edge of a petal may occasionally be found including one of the highest products of the plant, namely the pollen; for instance, I have seen in an Ophrys a pollen-mass with its curious structure of little packets, united together and to the caudicle by elastic threads, formed between the edges of an upper petal. The segments of the calyx of the common pea have been observed partially converted into carpels, including ovules, and with their tips converted into stigmas. Numerous analogous facts could be given.

I do not know how physiologists look at such facts as the foregoing. According to the doctrine of pangenesis, the free and superabundant gemmules of the transposed organs are developed in the wrong place, from uniting with wrong cells or aggregates of cells during their nascent state; and this would follow from a slight modification in the elective affinity of such cells, or possibly of certain gemmules. Nor ought we to feel much surprise at the affinities of cells and gemmules varying under domestication, when we remember the many curious cases given, in the seventeenth chapter, of cultivated plants which absolutely refuse to be fertilised by their own pollen or by that of the same species, but are abundantly fertile with pollen of a distinct species; for this implies that their sexual elective affinities—and this is the term used by Gärtner—have been modified. As the cells of adjoining or homologous parts will have nearly the same nature, they will be liable to acquire by variation each other's elective affinities; and we can thus to a certain extent understand such cases as a crowd of horns on the heads in certain sheep, of several spurs on the leg, and of hackles on the head of the fowl, and with the pigeon the occurrence of wing-feathers on their legs and of membrane between their toes; for the leg is the homologue of the wing. As all the organs of plants are homologous and spring from a common axis, it is natural that they should be eminently liable to transposition. It ought to be observed that when any compound part, such as an additional limb or an antenna, springs from a false position, it is only necessary that the few first gemmules should be wrongly attached; for these whilst developing would attract others in due succession, as in the regrowth of an amputated limb. When parts which are homologous and similar in structure, as the vertebræ in snakes or the stamens in polyandrous flowers, &c., are repeated many times in the same organism, closely allied gemmules must be extremely numerous, as well as the points to which they ought to become united; and, in accordance with the foregoing views, we can to a certain extent understand Isid. Geoffroy St. Hilaire's law, namely, that parts, which are already multiple, are extremely liable to vary in number.

The same general principles apply to the fusion of homologous parts; and with respect to mere cohesion there is probably always some degree of fusion, at least near the surface. When two embryos during their early development come into close contact, as both include corresponding gemmules, which must be in all respects almost identical in nature, it is not surprising that some derived from one embryo and some from the other should unite at the point of contact with a single nascent cell or aggregate of cells, and thus give rise to a single part or organ. For instance, two embryos might thus come to have on their adjoining sides a single symmetrical arm, which in one sense will have been formed by the fusion of the bones, muscles, &c., belonging to the arms of both embryos. In the case of the fish described by Lereboullet, in which a double head was seen gradually to fuse into a single one, the same process must have taken place, together with the absorption of all the parts which had been already formed. These cases are exactly the reverse of those in which a part is doubled either spontaneously or after an injury; for in the case of doubling, the superabundant gemmules of the same part are separately developed in union with adjoining points; whilst in the case of fusion the gemmules derived from two homologous parts become mingled and form a single part; or it may be that the gemmules from one of two adjoining embryos alone become developed.

Variability often depends, as I have attempted to show, on the reproductive organs being injuriously affected by changed conditions; and in this case the gemmules derived from the various parts of the body are probably aggregated in an irregular manner, some superfluous and others deficient. Whether a superabundance of gemmules, together with fusion during development, would lead to the increased size of any part cannot be told; but we can see that their partial deficiency, without necessarily leading to the entire abortion of the part, might cause considerable modifications; for in the same manner as a plant, if its own pollen be excluded, is easily hybridised, so, in the case of a cell, if the properly succeeding gemmules were absent, it would probably combine easily with other and allied gemmules. We see this in the case of imperfect nails growing on the stumps of amputated fingers, for the gemmules of the nails have manifestly been developed at the nearest point.

In variations caused by the direct action of changed conditions, whether of a definite or indefinite nature, as with the fleeces of sheep in hot countries, with maize grown in cold countries, with inherited gout, &c., the tissues of the body, according to the doctrine of pangenesis, are directly affected by the new conditions, and consequently throw off modified gemmules, which are transmitted with their newly acquired peculiarities to the offspring. On any ordinary view it is unintelligible how changed conditions, whether acting on the embryo, the young or adult animal, can cause inherited modifications. It is equally or even more unintelligible on any ordinary view, how the effects of the long-continued use or disuse of any part, or of changed habits of body or mind, can be inherited. A more perplexing problem can hardly be proposed; but on our view we have only to suppose that certain cells become at last not only functionally but structurally modified; and that these throw off similarly modified gemmules. This may occur at any period of development, and the modification will be inherited at a corresponding period; for the modified gemmules will unite in all ordinary cases with the proper preceding cells, and they will consequently be developed at the same period at which the modification first arose. With respect to mental habits or instincts, we are so profoundly ignorant on the relation between the brain and the power of thought that we do not know whether an inveterate habit or trick induces any change in the nervous system; but when any habit or other mental attribute, or insanity, is inherited, we must believe that some actual modification is transmitted; and this implies, according to our hypothesis, that gemmules derived from modified nerve-cells are transmitted to the offspring.

It is generally, perhaps always, necessary that an organism should be exposed during several generations to changed conditions or habits, in order that any modification in the structure of the offspring should ensue. This may be partly due to the changes not being at first marked enough to catch the attention, but this explanation is insufficient; and I can account for the fact, only by the assumption, which we shall see under the head of reversion is strongly supported, that gemmules derived from each cell before it had undergone the least modification are transmitted in large numbers to successive generations, but that the gemmules derived from the same cells after modification, naturally go on increasing under the same favouring conditions, until at last they become sufficiently numerous to overpower and supplant the old gemmules.

Another difficulty may be here noticed; we have seen that there is an important difference in the frequency, though not in the nature, of the variations in plants propagated by sexual and asexual generation. As far as variability depends on the imperfect action of the reproductive organs under changed conditions, we can at once see why seedlings should be far more variable than plants propagated by buds. We know that extremely slight causes,—for instance, whether a tree has been grafted or grows on its own stock, the position of the seeds within the capsule, and of the flowers on the spike,—sometimes suffice to determine the variation of a plant, when raised from seed. Now, it is probable, as explained when discussing alternate generation, that a bud is formed of a portion of already differentiated tissue; consequently an organism thus formed does not pass through the earlier phases of development, and cannot be so freely exposed, at the age when its structure would be most readily modified, to the various causes inducing variability; but it is very doubtful whether this is a sufficient explanation of the difficulty.

With respect to the tendency to reversion, there is a similar difference between plants propagated from buds and seed. Many varieties, whether originally produced from seed or buds, can be securely propagated by buds, but generally or invariably revert by seed. So, also, hybridised plants can be multiplied to any extent by buds, but are continually liable to reversion by seed,—that is, to the loss of their hybrid or intermediate character. I can offer no satisfactory explanation of this fact. Here is a still more perplexing case: certain plants with variegated leaves, phloxes with striped flowers, barberries with seedless fruit, can all be securely propagated by the buds on cuttings; but the buds developed from the roots of these cuttings almost invariably lose their character and revert to their former condition.

Finally, we can see on the hypothesis of pangenesis that variability depends on at least two distinct groups of causes. Firstly, on the deficiency, superabundance, fusion, and transposition of gemmules, and on the redevelopment of those which have long been dormant. In these cases the gemmules themselves have undergone no modification; but the mutations in the above respects will amply account for much fluctuating variability. Secondly, in the cases in which the organisation has been modified by changed conditions, the increased use or disuse of parts, or any other cause, the gemmules cast off from the modified units of the body will be themselves modified, and, when sufficiently multiplied, will be developed into new and changed structures.

Turning now to Inheritance: if we suppose a homogeneous gelatinous protozoon to vary and assume a reddish colour, a minute separated atom we aid naturally, as it grew to full size, retain the same colour; and we should have the simplest form of inheritance. Precisely the same view may be extended to the infinitely numerous and diversified units of which the whole body in one of the higher animals is composed; and the separated atoms are our gemmules. We have already sufficiently discussed the inheritance of the direct effects of changed conditions, and of increased use or disuse of parts, and, by implication, the important principle of inheritance at corresponding ages. These groups of facts are to a large extent intelligible on the hypothesis of pangenesis, and on no other hypothesis as yet advanced.

A few words must be added on the complete abortion or suppression of organs. When a part becomes diminished by disuse prolonged during many generations, the principle of economy of growth, as previously explained, will tend to reduce it still further; but this will not account for the complete or almost complete obliteration of, for instance, a minute papilla of cellular tissue representing a pistil, or of a microscopically minute nodule of bone representing a tooth. In certain cases of suppression not yet completed, in which a rudiment occasionally reappears through reversion, diffused gemmules derived from this part must, according to our view, still exist; hence we must suppose that the cells, in union with which the rudiment was formerly developed, in these cases fail in their affinity for such gemmules. But in the cases of complete and final abortion the gemmules themselves no doubt have perished; nor is this in any way improbable, for, though a vast number of active and long-dormant gemmules are diffused and nourished in each living creature, yet there must be some limit to their number; and it appears natural that gemmules derived from an enfeebled and useless rudiment would be more liable to perish than those derived from other parts which are still in full functional activity.

With respect to mutilations, it is certain that a part may be removed or injured during many generations, and no inherited result follow; and this is an apparent objection to the hypothesis which will occur to every one. But, in the first place, a being can hardly be intentionally mutilated during its early stages of growth whilst in the womb or egg; and such mutilations, when naturally caused, would appear like congenital deficiencies, which are occasionally inherited. In the second place, according to our hypothesis, gemmules multiply by self-division and are transmitted from generation to generation; so that during a long period they would be present and ready to reproduce a part which was repeatedly amputated. Nevertheless it appears, from the facts given in the twelfth chapter, that in some rare cases mutilations have been inherited, but in most of these the mutilated surface became diseased. In this case it may be conjectured that the gemmules of the lost part were gradually all attracted by the partially diseased surface, and thus perished. Although this would occur in the injured individual alone, and therefore in only one parent, yet this might suffice for the inheritance of a mutilation, on the same principle that a hornless animal of either sex, when crossed with a perfect animal of the opposite sex, often transmits its deficiency.

The last subject that need here be discussed, namely Reversion, rests on the principle that transmission and development, though generally acting in conjunction, are distinct powers; and the transmission of gemmules and their subsequent development show us how the existence of these two distinct powers is possible. We plainly see this distinction in the many cases in which a grandfather transmits to his grandson, through his daughter, characters which she does not, or cannot, possess. Why the development of certain characters, not necessarily in any way connected with the reproductive organs, should be confined to one sex alone—that is, why certain cells in one sex should unite with and cause the development of certain gemmules—we do not in the least know; but it is the common attribute of most organic beings in which the sexes are separate.

The distinction between transmission and development is likewise seen in all ordinary cases of Reversion; but before discussing this subject it may be advisable to say a few words on those characters which I have called latent, and which would not be classed under Reversion in its usual sense. Most, or perhaps all, the secondary characters, which appertain to one sex, lie dormant in the other sex; that is, gemmules capable of development into the secondary male sexual characters are included within the female; and conversely female characters in the male. Why in the female, when her ovaria become diseased or fail to act, certain masculine gemmules become developed, we do not clearly know, any more than why when a young bull is castrated his horns continue growing until they almost resemble those of a cow; or why, when a stag is castrated, the gemmules derived from the antlers of his progenitors quite fail to be developed. But in many cases, with variable organic beings, the mutual affinities of the cells and gemmules become modified, so that parts are transposed or multiplied; and it would appear that a slight change in the constitution of an animal, in connection with the state of the reproductive organs, leads to changed affinities in the tissues of various parts of the body. Thus, when male animals first arrive at puberty, and subsequently during each recurrent season, certain cells or parts acquire an affinity for certain gemmules, which become developed into the secondary masculine characters; but if the reproductive organs be destroyed, or even temporarily disturbed by changed conditions, these affinities are not excited. Nevertheless, the male, before he arrives at puberty, and during the season when the species does not breed, must include the proper gemmules in a latent state. The curious case formerly given of a Hen which assumed the masculine characters, not of her own breed but of a remote progenitor, illustrates the connexion between latent sexual characters and ordinary reversion. With those animals and plants which habitually produce several forms, as with certain butterflies described by Mr. Wallace, in which three female forms and the male exist, or as with the trimorphic species of Lythrum and Oxalis, gemmules capable of reproducing several widely-different forms must be latent in each individual.

The same principle of the latency of certain characters, combined with the transposition of organs, may be applied to those singular cases of butterflies and other insects, in which exactly one half or one quarter of the body resembles the male, and the other half or three quarters the female; and when this occurs the opposite sides of the body, separated from each other by a distinct line, sometimes differ in the most conspicuous manner. Again, these same principles apply to the cases given in the thirteenth chapter, in which the right and left sides of the body differ to an extraordinary degree, as in the spiral winding of certain shells, and as in the genus Verruca among cirripedes; for in these cases it is known that either side indifferently may undergo the same remarkable change of development.

Reversion, in the ordinary sense of the word, comes into action so incessantly, that it evidently forms an essential part of the general law of inheritance. It occurs with beings, however propagated, whether by buds or seminal generation, and sometimes may even be observed in the same individual as it advances in age. The tendency to reversion is often induced by a change of conditions, and in the plainest manner by the act of crossing. Crossed forms are generally at first nearly intermediate in character between their two parents; but in the next generation the offspring generally revert to one or both of their grandparents, and occasionally to more remote ancestors. How can we account for these facts? Each organic unit in a hybrid must throw off, according to the doctrine of pangenesis, an abundance of hybridised gemmules, for crossed plants can be readily and largely propagated by buds; but by the same hypothesis there will likewise be present dormant gemmules derived from both pure parent-forms; and as these latter retain their normal condition, they would, it is probable, be enabled to multiply largely during the lifetime of each hybrid. Consequently the sexual elements of a hybrid will include both pure and hybridised gemmules; and when two hybrids pair, the combination of pure gemmules derived from the one hybrid with the pure gemmules of the same parts derived from the other would necessarily lead to complete reversion of character; and it is, perhaps, not too bold a supposition that unmodified and undeteriorated gemmules of the same nature would be especially apt to combine. Pure gemmules in combination with hybridised gemmules would lead to partial reversion. And lastly, hybridised gemmules derived from both parent-hybrids would simply reproduce the original hybrid form. All these cases and degrees of reversion incessantly occur.

It was shown in the fifteenth chapter that certain characters are antagonistic to each other or do not readily blend together; hence, when two animals with antagonistic characters are crossed, it might well happen that a sufficiency of gemmules in the male alone for the reproduction of his peculiar characters, and in the female alone for the reproduction of her peculiar characters, would not be present; and in this case dormant gemmules derived from some remote progenitor might easily gain the ascendency, and cause the reappearance of long-lost characters. For instance, when black and white pigeons, or black and white fowls, are crossed,—colours which do not readily blend,—blue plumage in the one case, evidently derived from the rock-pigeon, and red plumage in the other case, derived from the wild jungle-cock, occasionally reappear. With uncrossed breeds the same result would follow, under conditions which favoured the multiplication and development of certain dormant gemmules, as when animals become feral and revert to their pristine character. A certain number of gemmules being requisite for the development of each character, as is known to be the case from several spermatozoa or pollen-grains being necessary for fertilisation, and time favouring their multiplication, will together account for the curious cases, insisted on by Mr. Sedgwick, of certain diseases regularly appearing in alternate generations. This likewise holds good, more or less strictly, with other weakly inherited modifications. Hence, as I have heard it remarked, certain diseases appear actually to gain strength by the intermission of a generation. The transmission of dormant gemmules during many successive generations is hardly in itself more improbable, as previously remarked, than the retention during many ages of rudimentary organs, or even only of a tendency to the production of a rudiment; but there is no reason to suppose that all dormant gemmules would be transmitted and propagated for ever. Excessively minute and numerous as they are believed to be, an infinite number derived, during a long course of modification and descent, from each cell of each progenitor, could not be supported or nourished by the organism. On the other hand, it does not seem improbable that certain gemmules, under favourable conditions, should be retained and go on multiplying for a longer period than others. Finally, on the views here given, we certainly gain some clear insight into the wonderful fact that the child may depart from the type of both its parents, and resemble its grandparents, or ancestors removed by many generations.

Conclusion.

The hypothesis of Pangenesis, as applied to the several great classes of facts just discussed, no doubt is extremely complex; but so assuredly are the facts. The assumptions, however, on which the hypothesis rests cannot be considered as complex in any extreme degree—namely, that all organic units, besides having the power, as is generally admitted, of growing by self-division, throw off free and minute atoms of their contents, that is gemmules. These multiply and aggregate themselves into buds and the sexual elements; their development depends on their union with other nascent cells or units; and they are capable of transmission in a dormant state to successive generations.

In a highly organised and complex animal, the gemmules thrown off from each different cell or unit throughout the body must be inconceivably numerous and minute. Each unit of each part, as it changes during development, and we know that some insects undergo at least twenty metamorphoses, must throw off its gemmules. All organic beings, moreover, include many dormant gemmules derived from their grandparents and more remote progenitors, but not from all their progenitors. These almost infinitely numerous and minute gemmules must be included in each bud, ovule, spermatozoon, and pollen-grain. Such an admission will be declared impossible; but, as previously remarked, number and size are only relative difficulties, and the eggs or seeds produced by certain animals or plants are so numerous that they cannot be grasped by the intellect.

The organic particles with which the wind is tainted over miles of space by certain offensive animals must be infinitely minute and numerous; yet they strongly affect the olfactory nerves. An analogy more appropriate is afforded by the contagious particles of certain diseases, which are so minute that they float in the atmosphere and adhere to smooth paper; yet we know how largely they increase within the human body, and how powerfully they act. Independent organisms exist which are barely visible under the highest powers of our recently-improved microscopes, and which probably are fully as large as the cells or units in one of the higher animals; yet these organisms no doubt reproduce themselves by germs of extreme minuteness, relatively to their own minute size. Hence the difficulty, which at first appears insurmountable, of believing in the existence of gemmules so numerous and so small as they must be according to our hypothesis, has really little weight.

The cells or units of the body are generally admitted by physiologists to be autonomous, like the buds on a tree, but in a less degree. I go one step further and assume that they throw off reproductive gemmules. Thus an animal does not, as a whole, generate its kind through the sole agency of the reproductive system, but each separate cell generates its kind. It has often been said by naturalists that each cell of a plant has the actual or potential capacity of reproducing the whole plant; but it has this power only in virtue of containing gemmules derived from every part. If our hypothesis be provisionally accepted, we must look at all the forms of asexual reproduction, whether occurring at maturity or as in the case of alternate generation during youth, as fundamentally the same, and dependent on the mutual aggregation and multiplication of the gemmules. The regrowth of an amputated limb or the healing of a wound is the same process partially carried out. Sexual generation differs in some important respects, chiefly, as it would appear, in an insufficient number of gemmules being aggregated within the separate sexual elements, and probably in the presence of certain primordial cells. The development of each being, including all the forms of metamorphosis and metagenesis, as well as the so-called growth of the higher animals, in which structure changes though not in a striking manner, depends on the presence of gemmules thrown off at each period of life, and on their development, at a corresponding period, in union with preceding cells. Such cells may be said to be fertilised by the gemmules which come next in the order of development. Thus the ordinary act of impregnation and the development of each being are closely analogous processes. The child, strictly speaking, does not grow into the man, but includes germs which slowly and successively become developed and form the man. In the child, as well as in the adult, each part generates the same part for the next generation. Inheritance must be looked at as merely a form of growth, like the self-division of a lowly-organised unicellular plant. Reversion depends on the transmission from the forefather to his descendants of dormant gemmules, which occasionally become developed under certain known or unknown conditions. Each animal and plant may be compared to a bed of mould full of seeds, most of which soon germinate, some lie for a period dormant, whilst others perish. When we hear it said that a man carries in his constitution the seeds of an inherited disease, there is much literal truth in the expression. Finally, the power of propagation possessed by each separate cell, using the term in its largest sense, determines the reproduction, the variability, the development and renovation of each living organism. No other attempt, as far as I am aware, has been made, imperfect as this confessedly is, to connect under one point of view these several grand classes of facts. We cannot fathom the marvellous complexity of an organic being; but on the hypothesis here advanced this complexity is much increased. Each living creature must be looked at as a microcosm—a little universe, formed of a host of self-propagating organisms, inconceivably minute and as numerous as the stars in heaven.