The Principles of Biology Vol. I/Chapter II.9

§ 85. Equally conspicuous with the truth that every organism bears a general likeness to its parents, is the truth that no organism is exactly like either parent. Though similar to both in generic and specific traits, and usually, too, in those traits which distinguish the variety, it diverges in numerous traits of minor importance. No two plants are indistinguishable; and no two animals are without differences. Variation is co-extensive with Heredity.

The degrees of variation have a wide range. There are deviations so small as to be not easily detected; and there are deviations great enough to be called monstrosities. In plants we may pass from cases of slight alteration in the shape of a leaf, to cases where, instead of a flower with its calyx above the seed-vessel, there is produced a flower with its calyx below the seed-vessel; and while in one animal there arises a scarcely noticeable unlikeness in the length or colour of the hair, in another an organ is absent or a supernumerary organ appears. Though small variations are by far the most general, yet variations of considerable magnitude are not uncommon; and even those variations constituted by additions or suppressions of parts, are not so rare as to be excluded from the list of causes by which organic forms are changed. Cattle without horns are frequent. Of sheep there are horned breeds and breeds that have lost their horns. At one time there existed in Scotland a race of pigs with solid feet instead of cleft feet. In pigeons, according to Mr. Darwin, "the number of the caudal and sacral vertebræ vary; as does the number of the ribs, together with their relative breadth and the presence of processes."

That variations, both small and large, which arise without any specific assignable cause, tend to become hereditary, was shown in the last chapter. Indeed the evidence which proves Heredity in its smaller manifestations is the same evidence which proves Variation; since it is only when there occur variations that the inheritance of anything beyond the structural peculiarities of the species can be proved. It remains here, however, to be observed that the transmission of variations is itself variable; and that it varies both in the direction of decrease and in the direction of increase. An individual trait of one parent may be so counteracted by the influence of the other parent, that it may not appear in the offspring; or, not being so counteracted, the offspring may possess it, perhaps in an equal degree or perhaps in a less degree; or the offspring may exhibit the trait in even a still higher degree. Among illustrations of this, one must suffice. I quote it from the essay by Sir J. Struthers referred to in the last chapter.

"The great-great-grandmother, Esther P—— (who married A—— L——), had a sixth little finger on one hand. Of their eighteen children (twelve daughters and six sons), only one (Charles) is known to have had digital variety. We have the history of the descendants of three of the sons, Andrew, Charles, and James.

"(1.) Andrew L—— had two sons, Thomas and Andrew; and Thomas had two sons all without digital variety. Here we have three successive generations without the variety possessed by the great-grandmother showing itself.

"(2.) James L——, who was normal, had two sons and seven daughters, also normal. One of the daughters became Mrs. J—— (one of the informants), and had three daughters and five sons, all normal except one of the sons, James J——, now æt. 17, who had six fingers on each hand....

"In this branch of the descendants of Esther, we see it passing over two generations and reappearing in one member of the third generation, and now on both hands.

"(3.) Charles L——, the only child of Esther who had digital variety, had six fingers on each hand. He had three sons, James, Thomas, and John, all of whom were born with six fingers on each hand, while John has also a sixth toe on one foot. He had also five other sons and four daughters, all of whom were normal.

"(a.) Of the normal children of this, the third generation, the five sons had twelve sons and twelve daughters, and the four daughters have had four sons and four daughters, being the fourth generation, all of whom were normal. A fifth generation in this sub-group consists as yet of only two boys and two girls who are also normal.

"In this sub-branch, we see the variety of the first generation present in the second, passing over the third and fourth, and also the fifth as far as it has yet gone.

"(b.) James had three sons and two daughters, who are normal.

"(c.) Thomas had four sons and five daughters, who are normal; and has two grandsons, also normal.

"In this sub-branch of the descent, we see the variety of the first generation, showing itself in the second and third, and passing over the fourth, and (as far as it yet exists) the fifth generation.

"(d.) John L—— (one of the informants) had six fingers, the additional finger being attached on the outer side, as in the case of his brothers James and Thomas. All of them had the additional digits removed. John has also a sixth toe on one foot, situated on the outer side. The fifth and sixth toes have a common proximal phalange, and a common integument invests the middle and distal phalanges, each having a separate nail.

"John L—— has a son who is normal, and a daughter, Jane, who was born with six fingers on each hand and six toes on each foot. The sixth fingers were removed. The sixth toes are not wrapped with the fifth as in her father's case, but are distinct from them. The son has a son and daughter, who, like himself, are normal.

"In this, the most interesting sub-branch of the descent, we see digital increase, which appeared in the first generation on one limb, appearing in the second on two limbs, the hands; in the third on three limbs, the hands and one foot; in the fourth on all the four limbs. There is as yet no fifth generation in uninterrupted transmission of the variety. The variety does not yet occur in any member of the fifth generation of Esther's descendants, which consists, as yet, only of three boys and one girl, whose parents were normal, and of two boys and two girls, whose grandparents were normal. It is not known whether in the case of the great-great-grandmother, Esther P——, the variety was original or inherited."

§ 86. Where there is great uniformity among the members of a species, the divergences of offspring from the average type are usually small; but where, among the members of a species, considerable unlikenesses have once been established, unlikenesses among the offspring are frequent and great. Wild plants growing in their natural habitats are uniform over large areas, and maintain from generation to generation like structures; but when cultivation has caused appreciable differences among the members of any species of plant, extensive and numerous deviations are apt to arise. Similarly, between wild and domesticated animals of the same species, we see the contrast that though the homogeneous wild race maintains its type with great persistence, the comparatively heterogeneous domestic race frequently produces individuals more unlike the average type than the parents are.

Though unlikeness among progenitors is one antecedent of variation, it is by no means the sole antecedent. Were it so, the young ones successively born to the same parents would be alike. If any peculiarity in a new organism were a direct resultant of the structural differences between the two organisms which produced it; then all subsequent new organisms produced by these two would show the same peculiarity. But we know that the successive offspring have different peculiarities: no two of them are ever exactly alike.

One cause of such structural variation in progeny, is functional variation in parents. Proof of this is given by the fact that, among progeny of the same parents, there is more difference between those begotten under different constitutional states than between those begotten under the same constitutional state. It is notorious that twins are more nearly alike than children borne in succession. The functional conditions of the parents being the same for twins, but not the same for their brothers and sisters (all other antecedents being constant), we have no choice but to admit that variations in the functional conditions of the parents, are the antecedents of those greater unlikenesses which their brothers and sisters exhibit.

Some other antecedent remains, however. The parents being the same, and their constitutional states the same, variation, more or less marked, still manifests itself. Plants grown from seeds out of one pod, or animals produced at one birth, are not alike. Sometimes they differ considerably. In a litter of pigs or of kittens, we rarely see uniformity of markings; and occasionally there are important structural contrasts. I have myself recently been shown a litter of Newfoundland puppies, some of which had four digits to their feet, while in others there was present, on each hind-foot, what is called the "dew-claw"—a rudimentary fifth digit.

Thus, induction points to three causes of variation, all in action together. We have heterogeneity among progenitors, which, did it act uniformly and alone in generating, by composition of forces, new deviations, would impress such new deviations to the same extent on all offspring of the same parents; which it does not. We have functional variation in the parents, which, acting either alone or in combination with the preceding cause, would entail the same structural variations on all young ones simultaneously produced; which it does not. Consequently there is some third cause of variation, yet to be found, which acts along with the structural and functional variations of ancestors and parents.

§ 87. Already, in the last section, there has been implied some relation between variation and the action of external conditions. The above-cited contrast between the uniformity of a wild species and the multiformity of the same species when cultivated or domesticated, thrusts this truth upon us. Respecting the variations of plants, Mr. Darwin remarks that "'sports' are extremely rare under nature, but far from rare under cultivation." Others who have studied the matter assert that if a species of plant which, up to a certain time, has maintained great uniformity, once has its constitution thoroughly disturbed, it will go on varying indefinitely. Though, in consequence of the remoteness of the periods at which they were domesticated, there is a lack of positive proof that our extremely variable domestic animals have become variable under the changed conditions implied by domestication, having been previously constant; yet competent judges do not doubt that this has been the case.

Now the constitutional disturbance which precedes variation, can be nothing else than an overthrowing of the pre-established equilibrium of functions. Transferring a plant from forest lands to a ploughed field or a manured garden, is altering the balance of forces to which it has been hitherto subject, by supplying it with different proportions of the assimilable matters it requires, and taking away some of the positive impediments to its growth which competing wild plants before offered. An animal taken from woods or plains, where it lived on wild food of its own procuring, and placed under restraint while artificially supplied with food not quite like what it had before, is an animal subject to new outer actions to which its inner actions must be adjusted. From the general law of equilibration we found it to follow that "the maintenance of such a moving equilibrium" as an organism displays, "requires the habitual genesis of internal forces corresponding in number, directions, and amounts, to the external incident forces—as many inner functions, single or combined, as there are single or combined outer actions to be met" (First Principles, § 173); and more recently ( § 27), we have seen that Life itself is "the definite combination of heterogeneous changes, both simultaneous and successive, in correspondence with external co-existences and sequences." Necessarily, therefore, an organism exposed to a permanent change in the arrangement of outer forces must undergo a permanent change in the arrangement of inner forces. The old equilibrium has been destroyed; and a new equilibrium must be established. There must be functional perturbations, ending in a re-adjusted balance of functions.

If, then, change of conditions is the only known cause by which the original homogeneity of a species is destroyed; and if change of conditions can affect an organism only by altering its functions; it follows that alteration of functions is the only known internal cause to which the commencement of variation can be ascribed. That such minor functional changes as parents undergo from year to year are influential on the offspring, we have seen is proved by the greater unlikeness that exists between children born to the same parents at different times, than exists between twins. And here we seem forced to conclude that the larger functional variations produced by greater external changes, are the initiators of those structural variations which, when once commenced in a species, lead by their combinations and antagonisms to multiform results. Whether they are or are not the direct initiators, they must still be the indirect initiators.

§ 87a. In the foregoing sentence those pronounced structural variations from which may presently arise new varieties and eventually species, are ascribed to "the larger functional variations produced by greater external changes"; and this limitation is a needful one, since there is a constant cause of minor variations of a wholly different kind.

There are the variations arising from differences in the conditions to which the germ is subject, both before detachment from the parent and after. At first sight it seems that plants grown from seeds out of the same seed-vessel and animals belonging to the same litter, ought, in the absence of any differences of ancestral antecedents, to be entirely alike. But this is not so. Inevitably they are subject from the very outset to slightly different sets of agencies. The seeds in a seed-vessel do not stand in exactly the same relations to the sources of nutriment: some are nearer than others. They are somewhat differently exposed to the heat and light penetrating their envelope; and some are more impeded in their growth by neighbours than others are. Similarly with young animals belonging to the same litter. Their uterine lives are made to some extent unlike by unlike connexions with the blood-supply, by mutual interferences not all the same, and even by different relations to the disturbances caused by the mother's movements. So, too, is it after separation from the parent plant or animal. Even the biblical parable reminds us that seeds fall into places here favourable and there unfavourable in various degrees. In respect of soil, in respect of space for growth, in respect of shares of light, none of them are circumstanced in quite the same ways. With animals the like holds. In a litter of pigs some, weaker than others, do not succeed as often in getting possession of teats. And then in both cases the differences thus initiated become increasingly pronounced. Among young plants the smaller, outgrown by their better-placed neighbours, are continually more shaded and more left behind; and among the litter the weakly ones, continually thrust aside by the stronger, become relatively more weakly from deficient nutrition.

Differentiations thus arising, both before and after separation from parents, though primarily differences of growth, entail structural differences; for it is a general law of nutrition that when there is deficiency of food the non-essential organs suffer more than the essential ones, and the unlikenesses of proportion hence arising constitute unlikenesses of structure. It may be concluded, however, that variations generated in this manner usually have no permanent results. In the first place, the individuals which, primarily in growth and secondarily in smaller developments of less-important organs, are by implication inferior, are likely to be eliminated from the species. In the second place, differences of structure produced in the way shown do not express differences of constitution—are not the effects of somewhat divergent physiological units; and consequently are not likely to be repeated in posterity.

§ 88. We have still, therefore, to explain those variations which have no manifest causes of the kinds thus far considered. These are the variations termed "spontaneous." Not that those who apply to them this word, or some equivalent, mean to imply that they are uncaused. Mr. Darwin expressly guards himself against such an interpretation. He says:—"I have hitherto sometimes spoken as if the variations—so common and multiform in organic beings under domestication, and in a lesser degree in those in a state of nature—had been due to chance. This, of course, is a wholly incorrect expression, but it serves to acknowledge plainly our ignorance of the cause of each particular variation." Not only, however, do I hold, in common with Mr. Darwin, that there must be some cause for these apparently-spontaneous variations, but it seems to me that a definite cause is assignable. I think it may be shown that unlikenesses must necessarily arise even between the new individuals simultaneously produced by the same parents. Instead of the occurrence of such variations being inexplicable, the absence of them would be inexplicable.

In any series of dependent changes a small initial difference often works a marked difference in the results. The mode in which a particular breaker bursts on the beach, may determine whether the seed of some foreign plant which it bears is or is not stranded—may cause the presence or absence of this plant from the Flora of the land; and may so affect, for millions of years, in countless ways, the living creatures throughout the land. A single touch, by introducing into the body some morbid matter, may set up an immensely involved set of functional disturbances and structural alterations. The whole tenor of a life may be changed by a word of advice; or a glance may determine an action which alters thoughts, feelings, and deeds throughout a long series of years. In those still more involved combinations of changes which societies exhibit, this truth is still more conspicuous. A hair's-breadth difference in the direction of some soldier's musket at the battle of Arcola, by killing Napoleon, might have changed events throughout Europe; and though the type of social organization in each European country would have been now very much what it is, yet in countless details it would have been different.

Illustrations like these, with which pages might be filled, prepare us for the conclusion that organisms produced by the same parents at the same time, must be more or less differentiated, both by insensible initial differences and by slight differences in the conditions to which they are subject during their evolution. We need not, however, rest with assuming such initial differences: the necessity of them is demonstrable. The individual germ-cells which, in succession or simultaneously, are separated from the same parent, can never be exactly alike; nor can the sperm-cells which fertilize them. When treating of the instability of the homogeneous (First Principles, § 149), we saw that no two parts of any aggregate can be similarly conditioned with respect to incident forces; and that being subject to forces that are more or less unlike, they must become more or less unlike. Hence, no two ova in an ovarium or ovules in a seed-vessel—no two spermatozoa or pollen-cells, can be identical. Whether or not there arise other contrasts, there are certain to arise quantitative contrasts; since the process of nutrition cannot be absolutely alike for all. The reproductive centres must begin to differentiate from the very outset. Such being the necessities of the case, what will happen on any successive or simultaneous fertilizations? Inevitably unlikenesses between the respective parental influences must result. Quantitative differences among the sperm-cells and among the germ-cells, will insure this. Grant that the number of physiological units contained in any one reproductive cell, can rarely if ever be exactly equal to the number contained in any other, ripened at the same time or at a different time; and it follows that among the fertilized germs produced by the same parents, the physiological units derived from them respectively will bear a different numerical ratio to each other in every case. If the parents are constitutionally quite alike, the variation in the ratio between the units they severally bequeath, cannot cause unlikenesses among the offspring. But if otherwise, no two of the offspring can be alike. In every case the small initial difference in the proportions of the slightly-unlike units, will lead, during evolution, to a continual multiplication of differences. The insensible divergence at the outset will generate sensible divergences at the conclusion. Possibly some may hence infer that though, in such case, the offspring must differ somewhat from each other and from both parents, yet that in every one of them there must result a homogeneous mixture of the traits of the two parents. A little consideration shows that the reverse is inferable. If, throughout the process of development, the physiological units derived from each parent preserved the same ratio in all parts of the growing organism, each organ would show as much as every other, the influence of either parent. But no such uniform distribution is possible. It has been shown (First Principles, § 163), that in any aggregate of mixed units segregation must inevitably go on. Incident forces will tend ever to cause separation of the two orders of units from each other—will tend to integrate groups of the one order in one place and groups of the other order in another place. Hence there must arise not a homogeneous mean between the two parents, but a mixture of organs, some of which mainly follow the one and some the other. And this is the kind of mixture which observation shows us.

Still it may be fairly objected that however the attributes of the two parents are variously mingled in their offspring, they must in all of them fall between the extremes displayed in the parents. In no characteristic could one of the young exceed both parents, were there no cause of "spontaneous variation" but the one alleged. Evidently, then, there is a cause yet unfound.

§ 89. Thus far we have contemplated the process under its simplest aspect. While we have assumed the two parents to be somewhat unlike, we have assumed that each parent has a homogeneous constitution—is built up of physiological units which are exactly alike. But in no case can such a homogeneity exist. Each parent had parents who were more or less contrasted—each parent inherited at least two orders of physiological units not quite identical. Here then we have a further cause of variation. The sperm-cells or germ-cells which any organism produces, will differ from each other not quantitatively only but qualitatively. Of the slightly-unlike physiological units bequeathed to it, the reproductive cells it casts off cannot habitually contain the same proportions; and we may expect the proportions to vary not slightly but greatly. Just as, during the evolution of an organism, the physiological units derived from the two parents tend to segregate, and produce likeness to the male parent in this part and to the female parent in that; so, during the formation of reproductive cells, there will arise in one a predominance of the physiological units derived from the father, and in another a predominance of the physiological units derived from the mother. Thus, then, every fertilized germ, besides containing different amounts of the two parental influences, will contain different kinds of influences—this having received a marked impress from one grandparent, and that from another. Without further exposition the reader will see how this cause of complication, running back through each line of ancestry, must produce in every germ numerous minute differences among the units.

Here, then, we have a clue to the multiplied variations, and sometimes extreme variations, that arise in races which have once begun to vary. Amid countless different combinations of units derived from parents, and through them from ancestors, immediate and remote—amid the various conflicts in their slightly-different organic polarities, opposing and conspiring with one another in all ways and degrees; there will from time to time arise special proportions causing special deviations. From the general law of probabilities it may be concluded that while these involved influences, derived from many progenitors, must, on the average of cases, obscure and partially neutralize one another; there must occasionally result such combinations of them as will produce considerable divergences from average structures; and, at rare intervals, such combinations as will produce very marked divergences. There is thus a correspondence between the inferable results and the results as habitually witnessed.

§ 90. Still there remains a difficulty. It may be said that admitting functional change to be the initiator of variation—granting that the physiological units of an organism long subject to new conditions, will tend to become modified in such way as to cause change of structure in offspring; yet there will still be no cause of the supposed heterogeneity among the physiological units of different individuals. There seems validity in the objection, that as all the members of a species whose circumstances have been altered will be affected in the same manner, the results, when they begin to show themselves in descendants, will show themselves in the same manner: not multiform variations will arise, but deviations all in one direction.

The reply is simple. The members of a species thus circumstanced will not be similarly affected. In the absence of absolute uniformity among them, the functional changes caused in them will be more or less dissimilar. Just as men of slightly-unlike dispositions behave in quite opposite ways under the same circumstances; or just as men of slightly-unlike constitutions get diverse disorders from the same cause, and are diversely acted on by the same medicine; so, the insensibly-differentiated members of a species whose conditions have been changed, may at once begin to undergo various kinds of functional changes. As we have already seen, small initial contrasts may lead to large terminal contrasts. The intenser cold of the climate into which a species has migrated, may cause in one individual increased consumption of food to balance the greater loss of heat; while in another individual the requirement may be met by a thicker growth of fur. Or, when meeting with the new foods which a new region furnishes, accident may determine one member of the species to begin with one kind and another member with another kind; and hence may arise established habits in these respective members and their descendants. Now when the functional divergences thus set up in sundry families of a species have lasted long enough to affect their constitutions, and to modify somewhat the physiological units thrown off in their reproductive cells, the divergences produced by these in offspring will be of divers kinds. And the original homogeneity of constitution having been thus destroyed, variation may go on with increasing facility. There will result a heterogeneous mixture of modifications of structure caused by modifications of function; and of still more numerous correlated modifications, indirectly so caused. By natural selection of the most divergent forms, the unlikenesses of parents will be rendered more marked, and the limits of variation wider. Until at length the divergences of constitutions and modes of life, become great enough to lead to segregation of the varieties.

§ 91. That variations must occur, and that they must ever tend, both directly and indirectly, towards adaptive modifications, are conclusions deducible from first principles; apart from any detailed interpretations like the above. That the state of homogeneity is an unstable state we have found to be a universal truth. Each species must pass from the uniform into the more or less multiform, unless the incidence of external forces is exactly the same for all its members, which it never can be. Through the process of differentiation and integration, which of necessity brings together, or keeps together, like individuals, and separates unlike ones from them, there must nevertheless be maintained a tolerably uniform species, so long as there continues a tolerably uniform set of conditions in which it may exist. But if the conditions change, either absolutely by some disturbance of the habitat or relatively by spread of the species into other habitats, then the divergent individuals that result must be segregated by the divergent sets of conditions into distinct varieties (First Principles, § 166). When, instead of contemplating a species in the aggregate, we confine our attention to a single member and its descendants, we see it to be a corollary from the general law of equilibration that the moving equilibrium constituted by the vital actions in each member of this family, must remain constant so long as the external actions to which they correspond remain constant; and that if the external actions are changed, the disturbed balance of internal changes, if not overthrown, cannot cease undergoing modification until the internal changes are again in equilibrium with the external actions: corresponding structural alterations having arisen.

On passing from these derivative laws to the ultimate law, we see that Variation is necessitated by the persistence of force. The members of a species inhabiting any area cannot be subject to like sets of forces over the whole of that area. And if, in different parts of the area, different kinds or amounts or combinations of forces act on them, they cannot but become different in themselves and in their progeny. To say otherwise, is to say that differences in the forces will not produce differences in the effects; which is to deny the persistence of force.