The Principles of Biology Vol. I/Chapter II.10

§ 92. A question raised, and hypothetically answered, in §§ 78 and 79, was there postponed until we had dealt with the topics of Heredity and Variation. Let us now resume the consideration of this question, in connexion with sundry others which the facts suggest.

After contemplating the several methods by which the multiplication of organisms is carried on—after ranging them under the two heads of Homogenesis, in which the successive generations are similarly produced, and Heterogenesis, in which they are dissimilarly produced—after observing that Homogenesis is nearly always sexual genesis, while Heterogenesis is asexual genesis with occasionally-recurring sexual genesis; we came to the questions—why is it that some organisms multiply in the one way and some in the other? and why is it that where agamogenesis prevails it is usually, from time to time, interrupted by gamogenesis? In seeking answers to these questions, we inquired whether there are common to both Homogenesis and Heterogenesis, any conditions under which alone sperm-cells and germ-cells arise and are united for the production of new organisms; and we reached the conclusion that, in all cases, they arise only when there is an approach to equilibrium between the forces which produce growth and the forces which oppose growth. This answer to the question—when does gamogenesis recur? still left unanswered the question—why does gamogenesis recur? And to this the reply suggested was, that the approach towards general equilibrium in organisms, "is accompanied by an approach towards molecular equilibrium in them; and that the need for this union of sperm-cell with germ-cell is the need for overthrowing this equilibrium, and re-establishing active molecular change in the detached germ—a result probably effected by mixing the slightly-different physiological units of slightly-different individuals." This is the hypothesis which we have now to consider. Let us first look at the evidences which certain inorganic phenomena furnish.

The molecules of any aggregate which have not a balanced arrangement, inevitably tend towards a balanced arrangement. As before mentioned (First Principles, § 100), amorphous wrought iron, when subject to continuous jar, begins to arrange itself into crystals—its atoms assume a condition of polar equilibrium. The particles of unannealed glass, which are so unstably arranged that slight disturbing forces make them separate into small groups, take advantage of that greater freedom of movement given by a raised temperature, to adjust themselves into a state of relative rest. During any such re-arrangement the aggregate exercises a coercive force over its units. Just as in a growing crystal the atoms successively assimilated from the solution, are made by the already crystallized atoms to take a certain form, and even to re-complete that form when it is broken; so in any mass of unstably-arranged atoms which passes into a stable arrangement, each atom conforms to the forces exercised on it by all the other atoms. This is a corollary from the general law of equilibration. We saw (First Principles, § 170) that every change is towards equilibrium; and that change can never cease until equilibrium is reached. Organisms, above all other aggregates, conspicuously display this progressive equilibration; because their units are of such kinds, and so conditioned, as to admit of easy re-arrangement. Those extremely active changes which go on during the early stages of evolution, imply an immense excess of the molecular forces over those antagonist forces which the aggregate exercises on the molecules. While this excess continues, it is expended in growth, development, and function: expenditure for any of these purposes being proof that part of the force constituting molecular tensions remains unbalanced. Eventually, however, this excess diminishes. Either, as in organisms which do not expend much energy, decrease of assimilation leads to its decline; or, as in organisms which expend much energy, it is counterbalanced by the rapidly-increasing reactions of the aggregate ( § 46). The cessation of growth when followed, as in some organisms, by death, implies the arrival at an equilibrium between the molecular forces and those forces which the aggregate opposes to them. When, as in other organisms, growth ends in the establishment of a moving equilibrium, there is implied such a decreased preponderance of the molecular forces, as leaves no surplus beyond that which is used up in functions. The declining functional activity characteristic of advancing life, expresses a further decline in this surplus. And when all vital movements come to an end, the implication is that the actions of the units on the aggregate and the reactions of the aggregate on the units are completely balanced. Hence, while a state of rapid growth indicates such a play of forces among the units of an aggregate as will produce active re-distribution, the diminution and arrest of growth shows that the units have fallen into such relative positions that re-distribution is no longer so facile. When, therefore, we see that gamogenesis recurs only when growth is decreasing, or has come to an end, we must say that it recurs only when the organic units are approximating to equilibrium—only when their mutual restraints prevent them from readily changing their arrangements in obedience to incident forces.

That units of like forms can be built up into a more stable aggregate than units of slightly unlike forms, is tolerably manifest à priori. And we have facts which prove that mixing allied but somewhat different units, does lead to comparative instability. Most metallic alloys exemplify this truth. Common solder, which is a mixture of lead and tin, melts at a much lower temperature than either lead or tin. The compound of lead, tin, and bismuth, called "fusible metal," becomes fluid at the temperature of boiling water; while the temperatures at which lead, tin, and bismuth become fluid are, respectively, 612°, 442°, and 497° F. Still more remarkable is the illustration furnished by potassium and sodium. These metals are very near akin in all respects—in their specific gravities, their atomic weights, their chemical affinities, and the properties of their compounds. That is to say, all the evidences unite to show that their units, though not identical, have a close resemblance. What now happens when they are mixed? Potassium alone melts at 136°, sodium alone melts at 190°, but the alloy of potassium and sodium is liquid at the ordinary temperature of the air. Observe the meaning of these facts, expressed in general terms. The maintenance of a solid form by any group of units implies among them an arrangement so stable that it is not overthrown by the incident forces. Whereas the assumption of a liquid form implies that the incident forces suffice to destroy the arrangement of the units. In the one case the thermal undulations fail to dislocate the parts; while in the other case the parts are so dislocated by the thermal undulations that they fall into total disorder—a disorder admitting of easy re-arrangement into any other order. For the liquid state is a state in which the units become so far free from mutual restraints, that incident forces can change their relative positions very readily. Thus we have reason to conclude that an aggregate of units which, though in the main similar to one another, have minor differences, must be more unstable than an aggregate of homogeneous units. The one will yield to disturbing forces which the other successfully resists.

Now though the colloidal molecules of which organisms are mainly built, are themselves highly composite; and though the physiological units compounded out of these colloidal molecules must have structures far more involved; yet it must happen with such units, as with simple units, that those which have exactly like forms will admit of arrangement into a more stable aggregate than those which have slightly-unlike forms. Among units of this order, as among units of a simpler order, imperfect similarity must entail imperfect balance in anything formed of them, and consequent diminished ability to withstand disturbing forces. Hence, given two organisms which, by diminished nutrition or increased expenditure, are being arrested in their growths—given in each an approaching equilibrium between the forces of the units and the forces of the aggregate—given, that is, such a comparatively balanced state among the units that re-arrangement of them by incident forces is no longer so easy; and it will follow that by uniting a group of units from the one organism with a group of slightly-different units from the other, the tendency towards equilibrium will be diminished, and the mixed units will be rendered more modifiable in their arrangements by the forces acting on them: they will be so far freed as to become again capable of that re-distribution which constitutes evolution.

And now let us test this hypothesis by seeing what power it gives us of interpreting established inductions.

§ 93. The majority of plants being hermaphrodites, it has, until quite recently, been supposed that the ovules of each flower are fertilized by pollen from the anthers of the same flower. Mr. Darwin, however, has shown that the arrangements are generally such as to prevent this. Either the ovules and the pollen are not ripe simultaneously, or obstacles prevent access of the one to the other. At the same time he has shown that there exist arrangements, often of a remarkable kind, which facilitate the transfer of pollen by insects from the stamens of one flower to the pistil of another. Similarly, it has been found that among the lower animals, hermaphrodism does not usually involve the production of fertile ova by the union of sperm-cells and germ-cells developed in the same individual; but that the reproductive centres of one individual are united with those of another to produce fertile ova. Either, as in Pyrosoma, Perophora, and in many higher molluscs, the ova and spermatozoa are matured at different times; or, as in annelids, they are prevented by their relative positions from coming in contact.

Remembering the fact that among the higher classes of organisms, fertilization is always effected by combining the sperm-cell of one individual with the germ-cell of another; and joining with it the above fact that among hermaphrodite organisms, the germ-cells developed in any individual are usually not fertilized by sperm-cells developed in the same individual; we see reason for thinking that the essential thing in fertilization, is the union of specially-fitted portions of different organisms. If fertilization depended on the peculiar properties of sperm-cell and germ-cell, as such; then, in hermaphrodite organisms, it would be a matter of indifference whether the united sperm-cells and germ-cells were those of the same individual or those of different individuals. But the circumstance that there exist in such organisms elaborate appliances for mutual fertilization, shows that unlikeness of derivation in the united reproductive centres, is the desideratum. Now this is just what the foregoing hypothesis implies. If, as was concluded, fertilization has for its object the disturbance of that approaching equilibrium existing among the physiological units separated from an adult organism; and if, as we saw reason to think, this object is effected by mixture with the slightly-different physiological units of another organism; then, we at the same time see that this object will not be effected by mixture with physiological units belonging to the same organism. Thus, the hypothesis leads us to expect such provisions as we find.

§ 94. But here a difficulty presents itself. These propositions seem to involve the conclusion that self-fertilization is impossible. It apparently follows from them, that a group of physiological units from one part of an organism ought to have no power of altering the state of approaching balance in a group from another part of it. Yet self-fertilization does occur. Though the ovules of one plant are generally fertilized by pollen from another plant of the same kind, yet they may be, some of them, fertilized by pollen of the same plant; and, indeed, there are plants in which self-fertilization is the rule: even provision being in some cases made to prevent fertilization by pollen from other individuals. And though, among hermaphrodite animals, self-fertilization is usually negatived by structural or functional arrangements, yet in certain Entozoa there appear to be special provisions by which the sperm-cells and the germ-cells of the same individual may be united, when not previously united with those of another individual. Nay, it has even been shown that in certain Ascidians the contents of oviduct and spermiduct of the same individual produce, when united, fertile ova whence evolve perfect individuals. Certainly, at first sight, these facts do not consist with the above supposition. Nevertheless there is something like a solution.

In the last chapter, when considering the variations caused in offspring from uniting elements representing unlike parental constitutions, it was pointed out that in an unfolding organism, composed of slightly-different physiological units derived from slightly-different parents, there cannot be maintained an even distribution of the two orders of units. We saw that the instability of the homogeneous negatives the uniform blending of them; and that, by the process of differentiation and integration, they must be more or less separated; so that in one part of the body the influence of one parent will predominate, and in another part of the body the influence of the other parent: an inference which harmonizes with daily observation. We also saw that the sperm-cells or germ-cells produced by such an organism must, in virtue of these same laws, be more or less unlike one another. It was shown that through segregation, some of the sperm-cells or germ-cells will get an excess of the physiological units derived from one side, and some of them an excess of those derived from the other side: a cause which accounts for the unlikenesses among offspring simultaneously produced. Now from this segregation of the different orders of physiological units, inherited from different parents and lines of ancestry, there arises the possibility of self-fertilization in hermaphrodite organisms. If the physiological units contained in the sperm-cells and germ-cells of the same flower, are not quite homogeneous—if in some of the ovules the physiological units derived from the one parent greatly predominate, and in some of the ovules those derived from the other parent; and if the like is true of the pollen-cells; then, some of the ovules may be nearly as much contrasted with some of the pollen-cells in the characters of their contained units, as were the ovules and pollen-cells of the parents from which the plant proceeded. Between part of the sperm-cells and part of the germ-cells, the community of nature will be such that fertilization will not result from their union; but between some of them, the differences of constitution will be such that their union will produce the requisite molecular instability. The facts, so far as they are known, seem in harmony with this deduction. Self-fertilization in flowers, when it takes place, is not so efficient as mutual fertilization. Though some of the ovules produce seeds, yet more of them than usual are abortive. From which, indeed, results the establishment of varieties that have structures favourable to mutual fertilization; since, being more prolific, these have, other things equal, greater chances in the "struggle for existence."

Further evidence is at hand supporting this interpretation. There is reason to believe that self-fertilization, which at the best is comparatively inefficient, loses all efficiency in course of time. After giving an account of the provisions for an occasional, or a frequent, or a constant crossing between flowers; and after quoting Prof. Huxley to the effect that among hermaphrodite animals, there is no case in which "the occasional influence of a distinct individual can be shown to be physically impossible;" Mr. Darwin writes—"from these several considerations and from the many special facts which I have collected, but which I am not here able to give, I am strongly inclined to suspect that, both in the vegetable and animal kingdoms, an occasional intercross with a distinct individual is a law of nature ... in none, as I suspect, can self-fertilization go on for perpetuity." This conclusion, based wholly on observed facts, is just the conclusion to which the foregoing argument points. That necessary action and the re-action between the parts of an organism and the organism as a whole—that power of an aggregate to re-mould the units, which is the correlative of the power of the units to build up into such an aggregate; implies that any differences existing among the units inherited by an organism, must gradually diminish. Being subject in common to the total forces of the organism, they will in common be modified towards congruity with these forces, and therefore towards likeness with one another. If, then, in a self-fertilizing organism and its self-fertilizing descendants, such contrasts as originally existed among the physiological units are progressively obliterated—if, consequently, there can no longer be a segregation of different physiological units in different sperm-cells and germ-cells; self-fertilization will become impossible. Step by step the fertility will diminish, and the series will finally die out.

And now observe, in confirmation of this view, that self-fertilization is limited to organisms in which an approximate equilibrium among the organic forces is not long maintained. While growth is actively going on, and the physiological units are subject to a continually-changing distribution of forces, no decided assimilation of the units can be expected: like forces acting on the unlike units will tend to segregate them, so long as continuance of evolution permits further segregation; and only when further segregation cannot go on, will the like forces tend to assimilate the units. Hence, where there is no prolonged maintenance of an approximate organic balance, self-fertilization may be possible for some generations; but it will be impossible in organisms distinguished by a sustained moving equilibrium.

§ 95. The interpretation which it affords of sundry phenomena familiar to breeders of animals, adds probability to the hypothesis. Mr. Darwin has collected a large "body of facts, showing, in accordance with the almost universal belief of breeders, that with animals and plants a cross between different varieties, or between individuals of the same variety but of another strain, gives vigour and fertility to the offspring; and on the other hand, that close interbreeding diminishes vigour and fertility,"—a conclusion harmonizing with the current belief respecting family-intermarriages in the human race. Have we not here a solution of these facts? Relations must, on the average of cases, be individuals whose physiological units are more nearly alike than usual. Animals of different varieties must be those whose physiological units are more unlike than usual. In the one case, the unlikeness of the units may frequently be insufficient to produce fertilization; or, if sufficient to produce fertilization, not sufficient to produce that active molecular change required for vigorous development. In the other case, both fertilization and vigorous development will be made probable.

Nor are we without a cause for the irregular manifestations of these general tendencies. The mixed physiological units composing any organism being, as we have seen, more or less segregated in the reproductive centres it throws off; there may arise various results according to the degrees of difference among the units, and the degrees in which the units are segregated. Of two cousins who have married, the common grandparents may have had either similar or dissimilar constitutions; and if their constitutions were dissimilar, the probability that their married grandchildren will have offspring will be greater than if their constitutions were similar. Or the brothers and sisters from whom these cousins descended, instead of severally inheriting the constitutions of their parents in tolerably equal degrees, may have severally inherited them in very different degrees: in which last case, intermarriages among the cousins will be less likely to prove infertile. Or the brothers and sisters from whom these cousins descended, may severally have married persons very like, or very unlike, themselves; and from this cause there may have resulted, either an undue likeness, or a due unlikeness, between the married cousins. These several causes, conspiring and conflicting in endless ways and degrees, will work multiform effects. Moreover, differences of segregation will make the reproductive centres produced by the same nearly-related organisms, vary considerably in their amounts of unlikeness; and therefore, supposing their amounts of unlikeness great enough to cause fertilization, this fertilization will be effective in various degrees. Hence it may happen that among offspring of nearly-related parents, there may be some in which the want of vigour is not marked, and others in which there is decided want of vigour. So that we are alike shown why in-and-in breeding tends to diminish both fertility and vigour: and why the effect cannot be a uniform effect, but only an average effect.

§ 96. While, if the foregoing arguments are valid, gamogenesis has for its main result the initiation of a new development by the overthrow of that approximate equilibrium arrived at among the molecules of the parent-organisms, a further result appears to be subserved by it. Those inferior organisms which habitually multiply by agamogenesis, have conditions of life that are simple and uniform; while those organisms which have highly-complex and variable conditions of life, habitually multiply by gamogenesis. Now if a species has complex and variable conditions of life, its members must be severally exposed to sets of conditions that are slightly different: the aggregates of incident forces cannot be alike for all the scattered individuals. Hence, as functional deviation must ever be inducing structural deviation, each individual throughout the area occupied tends to become fitted for the particular habits which its particular conditions necessitate; and in so far, unfitted for the average habits proper to the species. But these undue specializations are continually checked by gamogenesis. As Mr. Darwin remarks, "intercrossing plays a very important part in nature in keeping the individuals of the same species, or of the variety, true and uniform in character:" the idiosyncratic divergences obliterate one another. Gamogenesis, then, is a means of turning to positive advantage the individual differentiations which, in its absence, would result in positive disadvantage. Were it not that individuals are ever being made unlike one another by their unlike conditions, there would not arise in them those contrasts of molecular constitution, which we have seen to be needful for producing the fertilized germs of new individuals. And were not these individual differentiations ever being mutually cancelled, they would end in a fatal narrowness of adaptation.

This truth will be most clearly seen if we reduce it to its purely abstract form, thus:—Suppose a quite homogeneous species, placed in quite homogeneous conditions; and suppose the constitutions of all its members in complete concord with their absolutely-uniform and constant conditions; what must happen? The species, individually and collectively, is in a state of perfect moving equilibrium. All disturbing forces have been eliminated. There remains no force which can, in any way, change the state of this moving equilibrium; either in the species as a whole or in its members. But we have seen (First Principles, § 173) that a moving equilibrium is but a transition towards complete equilibration, or death. The absence of differential or un-equilibrated forces among the members of a species, is the absence of all forces which can cause changes in the conditions of its members—is the absence of all forces which can initiate new organisms. To say, as above, that complete molecular homogeneity existing among the members of a species, must render impossible that mutual molecular disturbance which constitutes fertilization, is but another way of saying that the actions and re-actions of each organism, being in perfect balance with the actions and re-actions of the environment upon it, there remains in each organism no force by which it differs from any other—no force which any other does not meet with an equal force—no force which can set up a new evolution among the units of any other.

And so we reach the remarkable conclusion that the life of a species, like the life of an individual, is maintained by the unequal and ever-varying actions of incident forces on its different parts. An individual homogeneous throughout, and having its substance everywhere continuously subject to like actions, could undergo none of those changes which life consists of; and similarly, an absolutely-uniform species, having all its members exposed to identical influences, would be deprived of that initiator of change which maintains its existence as a species. Just as, in each organism, incident forces constantly produce divergences from the mean state in various directions, which are constantly balanced by opposite divergences indirectly produced by other incident forces; and just as the combination of rhythmical functions thus maintained, constitutes the life of the organism; so, in a species, there is, through gamogenesis, a perpetual neutralization of those contrary deviations from the mean state which are caused in its different parts by different sets of incident forces; and it is similarly by the rhythmical production and compensation of these contrary deviations, that the species continues to live. The moving equilibrium in a species, like the moving equilibrium in an individual, would rapidly end in complete equilibration, or death, were not its continually-dissipated forces continually re-supplied from without. Besides owing to the external world those energies which, from moment to moment, keep up the lives of its individual members, every species owes to certain more indirect actions of the external world, those energies which enable it to perpetuate itself in successive generations.

§ 97. What evidence still remains may be conveniently woven up along with a recapitulation of the argument pursued through the last three chapters. Let us contemplate the facts in their synthetic order.

That compounding and re-compounding through which we pass from the simplest inorganic substances to the most complex organic substances, has several concomitants. Each successive stage of composition presents us with molecules that are severally larger or more integrated, that are severally more heterogeneous, that are severally more unstable, and that are more numerous in their kinds (First Principles, § 151). And when we come to the substances of which living bodies are formed, we find ourselves among innumerable divergent groups and sub-groups of compounds, the units of which are large, heterogeneous, and unstable, in high degrees. There is no reason to assume that this process ends with the formation of those complex colloids which constitute organic matter. A more probable assumption is that out of the complex colloidal molecules there are evolved, by a still further integration, molecules which are still more heterogeneous, and of kinds which are still more multitudinous. What must be their properties? Already the colloidal molecules are extremely unstable—capable of being variously modified in their characters by very slight incident forces; and already the complexity of their polarities prevents them from readily falling into such positions of equilibrium as results in crystallization. Now the organic molecules composed of these colloidal molecules, must be similarly characterized in far higher degrees. Far more numerous must be the minute changes that can be wrought in them by minute external forces; far more free must they remain for a long time to obey forces tending to re-distribute them; and far greater must be the number of their kinds.

Setting out with these physiological units, the existence of which various organic phenomena compel us to recognize, and the production of which the general law of Evolution thus leads us to anticipate; we get an insight into the phenomena of Genesis, Heredity, and Variation. If each organism is built of certain of these highly-plastic units peculiar to its species—units which slowly work towards an equilibrium of their complex proclivities, in producing an aggregate of the specific structure, and which are at the same time slowly modifiable by the re-actions of this aggregate—we see why the multiplication of organisms proceeds in the several ways, and with the various results, which naturalists have observed.

Heredity, as shown not only in the repetition of the specific structure but in the repetition of ancestral deviations from it, becomes a matter of course; and it falls into unison with the fact that, in various inferior organisms, lost parts can be replaced, and that, in still lower organisms, a fragment can develop into a whole.

While an aggregate of physiological units continues to grow by the assimilation of matter which it moulds into other units of like type; and while it continues to undergo changes of structure; no equilibrium can be arrived at between the whole and its parts. Under these conditions, then, an un-differentiated portion of the aggregate—a group of physiological units not bound up into a specialized tissue—will be able to arrange itself into the structure peculiar to the species; and will so arrange itself, if freed from controlling forces and placed in fit conditions of nutrition and temperature. Hence the continuance of agamogenesis in little-differentiated organisms, so long as assimilation continues to be greatly in excess of expenditure.

But let growth be checked and development approach its completion—let the units of the aggregate be severally exposed to an almost constant distribution of forces; and they must begin to equilibrate themselves. Arranged, as they will gradually be, into comparatively stable attitudes in relation to one another, their mobility will diminish; and groups of them, partially or wholly detached, will no longer readily re-arrange themselves into the specific form. Agamogenesis will be no longer possible; or, if possible, will be no longer easy.

When we remember that the force which keeps the Earth in its orbit is the gravitation of each particle in the Earth towards every one of the group of particles existing 92,000,000 of miles off; we cannot reasonably doubt that each unit in an organism acts on all the other units, and is reacted on by them: not by gravitation only but chiefly by other energies. When, too, we learn that glass has its molecular constitution changed by light, and that substances so rigid and stable as metals have their atoms re-arranged by forces radiated in the dark from adjacent objects; we are obliged to conclude that the excessively-unstable units of which organisms are built, must be sensitive in a transcendant degree to all the forces pervading the organisms composed of them—must be tending ever to re-adjust, not only their relative attitudes but their molecular structures, into equilibrium with these forces. Hence, if aggregates of the same species are differently conditioned, and re-act differently on their component units, their component units will be rendered somewhat different; and they will become the more different the more widely the re-actions of the aggregates upon them differ, and the greater the number of generations through which these different re-actions of the aggregates upon them are continued.

If, then, unlikenesses of function among individuals of the same species, produce unlikenesses between the physiological units of one individual and those of another, it becomes comprehensible that when groups of units derived from two individuals are united, the group formed will be more unstable than either of the groups was before their union. The mixed units will be less able to resist those re-distributing forces which cause evolution; and may thus have restored to them the capacity for development which they had lost.

This view harmonizes with the conclusion, which we saw reason to draw, that fertilization does not depend on any intrinsic peculiarities of sperm-cells and germ-cells, but depends on their derivation from different individuals. It explains the facts that nearly-related individuals are less likely to have offspring than others, and that their offspring, when they have them, are frequently feeble. And it gives us a key to the converse fact that the crossing of varieties results in unusual vigour.

Bearing in mind that the slightly-different orders of physiological units which an organism inherits from its parents, are subject to the same set of forces, and that when the organism is fully developed this set of forces, becoming constant, tends slowly to re-mould the two orders of units into the same form; we see how it happens that self-fertilization becomes impossible in the higher organisms, while it remains possible in the lower organisms. In long-lived creatures which have tolerably-definite limits of growth, this assimilation of the somewhat-unlike physiological units is liable to go on to an appreciable extent; whereas in organisms which do not continuously subject their component units to constant forces, there will be much less of this assimilation. And where the assimilation is not considerable, the segregation of mixed units may cause the sperm-cells and germ-cells developed in the same individual, to be sufficiently different to produce, by their union, fertile germs; and several generations of self-fertilizing descendants may succeed one another, before the two orders of units have had their unlikenesses so far diminished that they will no longer do this. The same principles explain for us the variable results of union between nearly-related organisms. According to the contrasts among the physiological units they inherit from parents and ancestors; according to the unlike proportions of the contrasted units which they severally inherit; and according to the degrees of segregation of such units in different sperm-cells and germ-cells; it may happen that two kindred individuals will produce the ordinary number of offspring or will produce none; or will at one time be fertile and at another not; or will at one time have offspring of tolerable strength and at another time feeble offspring.

To the like causes are also ascribable the phenomena of Variation. These are unobtrusive while the tolerably-uniform conditions of a species maintain tolerable uniformity among the physiological units of its members; but they become obtrusive when differences of conditions, entailing considerable functional differences, have entailed decided differences among the physiological units, and when the different physiological units, differently mingled in every individual, come to be variously segregated and variously combined.

Did space permit, it might be shown that this hypothesis is a key to many further facts—to the fact that mixed races are comparatively plastic under new conditions; to the fact that pure races show predominant influences in the offspring when crossed with mixed races; to the fact that while mixed breeds are often of larger growth, pure breeds are the more hardy—have functions less-easily thrown out of balance. But without further argument it will, I think, be admitted that the power of this hypothesis to explain so many phenomena, and to bring under a common bond phenomena which seem so little allied, is strong evidence of its truth. And such evidence gains greatly in strength on observing that this hypothesis brings the facts of Genesis, Heredity, and Variation into harmony with first principles. We see that these plastic physiological units, which we find ourselves obliged to assume, are just such more integrated, more heterogeneous, more unstable, and more multiform molecules, as would result from continuance of the steps through which organic matter is reached. We see that the differentiations of them assumed to occur in differently-conditioned aggregates, and the equilibrations of them assumed to occur in aggregates which maintain constant conditions, are but corollaries from those universal principles implied by the persistence of force. We see that the maintenance of life in the successive generations of a species, becomes a consequence of the continual incidence of new forces on the species, to replace the forces that are ever being rhythmically equilibrated in the propagation of the species. And we thus see that these apparently-exceptional phenomena displayed in the multiplication of organic beings, fall into their places as results of the general laws of Evolution. We have, therefore, weighty reasons for entertaining the hypothesis which affords us this interpretation.