§ 88. That they are not in all cases, or even in most cases, the direct initiators, is clear; Were they so, those unlike- nesses which exist between plants that grow from seeds out of the same seed-vessel, or between animals belonging to the same litter, would be inexplicable. Here, all the antecedents, structural and functional, appear to be alike for each of the new organisms. Any deviations caused by structural con- trasts or functional disturbances in the parents, must be equally shared in by all simultaneously-produced offspring. Hence, an explanation of the variations arising under such conditions, has still to be sought.
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 domestica- tion, 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, how- ever, 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 ix definite cause is assignable. I think it may be shown that unlikenesses must necessarily arise between the new individuals simultaneously produced by the same parents. Instead of the occurrence of such VAUIATION. 265 variations being inexplicable, we shall presently see that 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 modi3 in which a particular breaker bursts on the beach, may determine whether the seed of some foreign plant which it bears, is oris 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 mat- ter, may set up an immensely- involved set of functional dis- turbances 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 differ- ence in the direction of some soldier's musket at the battle of Areola, by killing Napoleon, might have changed events throughout Europe: though the 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 succes- sion 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 PrincipleSy g 109), we saw that no two parts of any aggregate, can be similarly conditioned with 266 THE INDUCTIONS OF BIOLOGY.
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 ovarum 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 repro- ductive centres must begin to differentiate from the very outset. Such being the necessities of the case, what will happen on any successiv^e or simultaneous fertilizations? There will inevitably result more or less unlikeness between the combined parental influences in every instance. Quan- titative 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 each parent will bear a different numerical ratio to each other in every case. If now the parents are constitutionally alike, that is, alike in the polarities of their physiological units, the variation in the ratio between the physiological units they severally bequeath to the fertilized germs, 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 gener- ate 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 VARIATION. 2G7 units derived from each parent, preserved the same ratio to each other in all parts of the growing organism, each organ would show as much as every other, the influence of eitlicr parent. But we know, a j)riori, that no such uniform dis- tribution is possible. It has been shown [First PrinciplcSy § 123), that in any mixed aggregate of units, segregation must inevitably go on. Incident forces will tend ever to cause separation of the two orders of units from each other — will 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 parent and some the other. And this is the kind of mixture which observation shows us.
Still it may be fliirly objected, that however the attributes of the two parents are variously mixed in their several offspring, they must in all tlie offspring 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. Evi- dently, 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 physiologi - cal units that are exactly alike. But in no case can such a homogeneity exist. Each parent had parents that 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 an organism, its reproductive cells cannot habitually contain the same pro- portions; and we may expect the proportions to vary not 268 THE INDUCTIONS OF BIOLOGY.
slightly but greatly. Just as, during the evolution of an or- ganism, the physiological units derived from the two parents tend to segregate, and produce likeness to the male parent in this feature and to the female parent in that; so, during the formation of reproductive cells by such organism, there will arise in one cell a predominance of the physiological units derived from one parent, and in another cell a predominance of the physiological units derived from the other parent. The instability of the homogeneous forbids us to assume an even distribution of the two orders of units in all the reproductive cells. And inequalities once arising among them, must tend ever to become more marked; since, wherever units of a given order have begun to segregate, the process of differenti- ation and integration tends to segregate them more and more. 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 maternal or paternal ancestor, and that from an- other.
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 ances- tors, immediate and remote — amid the various conflicts in their slightly-different polarities, opposing and conspiring with each other in all ways and degrees; there will from time to time arise special proportions causing special devi- ations. From the general law of probabilities it is inferable, that while these involved influences, derived from many pro- genitors, 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 com- binations as will produce very marked divergences. Th^re is thus a correspondence between the inferable results, and the results as habitually witnessed.
VARIATION. 2(\^ VARIATION. 2(\^ § 90. Still there remains a difficulty. It may bo said that admitting functional change to bo the initiator of variation — granting that the physiological units of an organism, modified by long subjection to new conditions, will tend to be- come 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 in- dividuals. There seems validity in the objection, that as all the members of a species whose circumstances have been al- tered, will be affected in the same manner, the results, when they begin to show themselves in descendants, will show them- selves 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 cir- cumstanced, 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 con- ditions 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 con- trasts. The in tenser cold of the climate into which a species has migrated, may cause in one individual increased con- sumption of food, to balance the greater loss of heat; while in another individual, the new requirement may be met by a thicker growth of fur. Or, when meeting with the new foods which the new region furnishes, mere accident may deter- mine one member of the species to begin with one kind and another member with another kind; aud 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 270 THE INDUCTIONS OF BIOLOGY.
to affect their constitutions profoundly, and to modify somc- wliat tlie physiological units thrown off in their reproductive cells, the divergences produced by these in offspring, will be of diverse kinds. And the original homogeneity of constitu- tion having been thus destroyed, variation may go on with increasing facility. There will result a heterogeneous mix- ture of modifications of structure, caused by modifications of function; and of still more numerous correlated modifica- tions, indirectly so caused. By natural selection of the most divergent forms, the unlikenesses of parents will grow 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 modifica- tions, 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 uni- form 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 to- gether, 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, § 126). 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 equili- VARIATION. 271 brium constituted by the vital actions in each member of this family, must remain constant so long as the external ac- tions 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 equili- brium with the external actions: corresponding structural alterations having arisen.
Or 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 aggregates 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 differ- ences in the effects; which is to deny the persistence of force.
Whence it is also manifest, that there can be no variation of structure, but what is directly or indirectly consequent on variation of function. On the one hand, organisms in com- plete equilibrium with their conditions, cannot be changed except by change in their conditions; since, to assert other- wise, is to assert that there can be an effect without a cause; which is to deny the persistence of force. On the other hand, any change of conditions can affect an organism only by changing the actions going on in it — only b}" altering its func- tions. The alterations of functions baing necessarily towards a re-establishment of the equilibrium, (for if not, the equili- brium must be destroyed and the life cease, either in the in- dividual or in descendants,) it follows that the structural alter- ations directly caused, are adaptations; and that the correlated structural alterations indirectly caused, are the concomitants of adaptations. Hence, though, by the intercourse of organisms that have been functionally and structurally modified in dif- ferent directions, there may result organisms that deviate in compound ways which appear unrelated to external condi- 272 THE INDUCTIONS OF BIOLOGY.
tions, the deviations of such organisms must still be regarded as indirect results of functional adaptations. We must say that in all cases, adaptive change of function is the primary and ever-acting cause of that change of structure which con- stitutes variation; and that the variation which appears to be " spontaneous," is derivative and secondary.
CHAPTER X.
GENESIS, HEREDITY, AND VARIATION.
§ 92. A QUESTION raised, and hypotlietlcally 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 suc- cessive generations are similarly produced, and Heterogenesis, in which they are dissimilarly produced — after observing that Homogenesis is always sexual genesis, while Heteroge- nesis is asexual genesis with occasionally-recurring sexual genesis; we came to the questions — why is it that some or- ganisms 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 an answer to this question, we inquired whether there are, common to both Homogenesis and Fieterogenesis, any condi- tions 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?
18 274 THE INDUCTIONS OF BIOLOGY.
still left unanswered the question — ivhy does gamogenesis recur? And to this the reply suggested was, that the ap- proach towards general equilibrium in organisms, " is ac- companied by an approach towards molecular equilibrium in them; and that the need for this union of sperm-cell and germ-cell, is the need for overthrowing this equilibrium, and re-establishing active molecular change in the detached germ — a result which is 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 pheno- mena furnish.
The molecules of any aggregate which have not a balanced arrangement, inevitably tend towards a balanced arrangement. As before mentioned [First Principles, § 103) 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 ad- just 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 suc- cessively assimilated from the solution, are made by the al- ready-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 that 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, § 130) 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 GENESIS, HEREDITY, AND VARIATION. 275 extremely active changes which go on during the early stages of evolution, imply an immense excess of the mole- cular forces over those antagonist forces which the aggregate exercises on the molecules While this excess continues, it is expended in growth, development, and function — expendi- ture for any of these purposes, being proof that part of the force embodied in molecular tensions, remains unbalanced. Eventually, however, this excess diminishes. Either, as in organisms which do not expend much force, decrease of assi- milation leads to its decline; or, as in organisms which ex- pend much force, it is counterbalanced by the rapidly-increas- ing re-actions of the aggregate (§ 46). The cessation of growth, when followed, as in some organisms, by death, im- plies 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 establish- ment of a moving equilibrium, there is implied such a de- creased preponderance of the molecular forces, as leaves no surplus beyond that which is used up in functions. The de- clining 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 re- actions of the aggregate on the units, are completely bal- anced. Hence, while a state of rapid growth indi- cates 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 approxima- ting to equilibrium — only when their mutual restraints pre- vent them from readily changing their arrangements in obe- dience to incident forces.
That units of like forms can be built up into a moie stable 276 THE INDUGTIOINS OF BIOLOGY.
aggregate thaii units of slightly unlike forms, is tolerably manifest, a priori. And we have facts which prove that mixing allied but somewhat different units, o?oes lead to comparative in- stability. Most metallic alloys exemplify this truth. Com- mon 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 illustra- tion furnished by potassium and sodium. These metals are very near akin in all respects — in their specific gravities, their atomic weights, their chemical afiinities, 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 resem- blance. What now happens when t\\Qj 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 cannot be overthrown by the incident forces. Whereas the assumption of a liquid form, implies that the incident forces sufiice 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 each other, 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.
OENKSTS, HEREDITY, AND VARTATTON. 277