Thus the law of the instability of the homogeneous, has here a three-fold coroUarj^ As interpreted in connexion with the ever-progressing, ever-complicating changes in external fac- tors, it brrugs us to the conclusion that there must be a pre- vailing tendency towards greater heterogeneity in all kinds of organisms, considered both individually and in successive generations; as well as in each assemblage of organisms con- stituting a species; and, by consequence, in each genus, order, and class.
§ 155. When considering the causes of evolution in general, we further saw {First Principles, § 116), that the multiplication of effects aids continually to increase that heterogeneity into which homogeneity inevitably lapses. It was pointed out that since " the several parts of an aggre- gate are differently modified by any incident force; '^ and that since " by the reactions of the differently modified parts the incident force itself must be divided into differently modified parts; " it follows that ^' each differentiated di- vision of the aggregate thus becomes a centre from which a differentiated division of the original force is again diffused. And since unlike forces must produce unlike results, each of these differentiated forces must produce, throughout the aggregate, a further series of differentia- tions." And to this it was added, that in proportion as 424 THE EVOLUTION OF LIFE.
the heterogeneity increases, the complications arising from this multiplication of effects grow more marked; since the more strongly contrasted the parts of an aggregate become, the more different must be their reactions upon incident forces, and the more unlike must be the secondary sets of effects which these modified incident forces initiate; and since every increase in the number of unlike parts increases the number of such differentiated incident forces, and such secondary sets of effects.
How this multiplication of effects conspires with the in- stability of the homogeneous, to work an increasing multi- formity of structure in an organism, was shown at the time; and the foregoing pages contain further incidental illustra- tions. Under the head of Adaptation (§ 69), it was show^n that a change in one function must act and re-act through ever-compKcating perturbations on the rest; and that, eventu- ally, all parts of the organism must be modified in their states. Suppose that the head of a mammal becomes very much more weighty — what must be the indirect results? The muscles of the neck are put to greater exertions; and its vertebrae have to bear additional tensions and pressures, caused both by the increased weight of the head, and the stronger contractions of the muscles that support and move the head. These muscles also affect their own attachments: several of the dorsal spines have augmented strains put on them; and the vertebrae to which they are fixed, are more severely taxed. Further, this heavier head and the more massive neck it necessitates, require a stronger fulcrum: the whole thoracic arch, and the fore limbs which support it, are sub- ject to greater continuous stress and more violent occasional shocks. And the required strengthening of the fore-quarters cannot take place, without, the centre of gravity being changed, and the hind limbs being differently reacted upon during locomotion. Any one who compares the outline of the bison with that of its congener, the ox, will clearly see how profoundly a heavier head affects the entire osseous INTERNAL FACTORS. 425 and muscular systems. Besides this multiplication of mcclianical effects, there is a multiplication of physiological effects. The vascular apparatus is modified throughout its whole structure, by each considerable modifi- cation in the proportions of the body. Increase in the size of any organ, implies a quantitative, and often a qualitative, reaction on the blood; and so alters the nutrition of all other organs. Such physiological correlations are exemplified in the many differences that accompany difference of sex. That the minor sexual peculiarities are brought about by the physio- logical actions and reactions, is shown both by the fact that they are commonly but faintly marked until the fundamentally distinctive organs are developed; and that when the de- velopment of these is prevented, the minor sexual peculiarities do not arise. No further proof is, I think, needed, that in any individual organism or its descendants, a new external action must, besides the primary internal change which it works, work sundry secondary changes, as well as tertiary changes still more multiplied. That tendency to- wards greater heterogeneity which is given to an organ- ism by disturbing its environment, is helped by the tendency which every modification "has to produce other modifications — modifications which must become more numerous in pro- portion as the organism becomes more complex. And then, lastly, among the indirect and involved manifestations of this tendency, we must not omit the innumerable small irregularities of structure that result from the crossing of dissimilarly-modified individuals. It was shoTSTi (§§ 89, 90) that what are called "spontaneous variations," are inter- pretable as results of miscellaneously compounding the changes wrought in different lines of ancestors by different conditions of life. These still more complex and multi- tudinous effects so produced, are thus further illustrations of the multiplication of effects.
Equally in the aggregate of individuals constituting a species, does multiplication of effects become the continual 426 THE EVOLUTION OF LIFE.
cause of increasing multiformity. The lapse of a species into divergent varieties, initiates fresh combinations of forces tending to work further divergences. The new varieties compete with the parent species in new ways; and so add new elements to its circumstances. They modify somewhat the conditions of other species existing in their habitat, or into w^hose habitat they have spread; and the modifications wrought in such other species, become additional sources of influence. The Flora and Fauna of every region are united by their entangled relations into a whole, of which no part can be afiected without afiecting the rest. Hence, each dif- ferentiation in a local assemblage of species, becomes the cause of further difierentiations in such assemblage.
§ 156. One of the universal principles to which we saw that the re-distribution of matter and motion conforms, is that in any aggregate made up of mixed units, incident forces produce segregation — separate unlike units and unite like units; and it was shown that the increasing integration and definiteness which characterizes each part of an evolving organic aggregate, as of every other aggregate, results from this {First Prhicijples, § 126). It remains here to be pointed out, that while the actions and reactions going on between organisms and their ever- changing environments, add to the heterogeneity of organic structures, they also give to the heterogeneity this growing distinctness. At first sight the reverse might be inferred. It might be argued that any new set of efiects wrought in an organism by some new set of external forces, must tend more or less to obliter- ate the effects previously wrought — must produce confusion or indefiniteness. A little consideration, however, will dissi- pate this impression.
Doubtless the condition under which alone increasing de- finiteness of structure can be acquired by any part of an or- ganism, either in an individual or in successive generations, is that such part shall be exposed to some set of tolerably-con- INTERNAL FACTORS. 427 stant forces; and doubtless, continual change of circumstances interferes with this. But the interference can never be con- siderable. For the pre-existing structure of an organism pre- vents it from living under any new conditions except such as are congruous with the fundamental characters of its organiza- tion— such as subject its essential organs to actions substan- tially the same as before. Great changes must kill it. Hence, it can continuously expose itself ^ and its descendants, only to those moderate changes which do not destroy the general har- mony between the aggregate of incident forces and the ag- gregate of its functions. That is, it must remain under influences calculated to make greater the definiteness of the chief differentiations already produced. If, rfor ex- ample, we set out with an animal in which a rudimentary vertebral column with its attached muscular system has been established; it is clear that the mechanical arrange- ments have become thereby so far determined, that sub- sequent modifications are extremely likely, if not certain, to be consistent with the production of movement by the action of muscles on a flexible central axis. Hence, there will con- tinue a general similarity in the play of forces to which the flexible central axis is subject; and so, notwithstanding the metamorphoses which the vertebrate type undergoes, there "will be a maintenance of conditions favourable to increasing definiteness and integration of the vertebral column. More- over, this maintenance of such conditions becomes secure in proportion as organization advances. Each further com- plexity of structure, implying some further complexity in the relations between an organism and its environment, must tend to specialize the actions and reactions between it and its environment — must tend to increase the stringency with which it is restrained within such environments as admit of those special actions and reactions for which its structure fits it; that is, must further guarantee the continuance of those actions and reactions to which its essential organs respond, and therefore the continuance of the segregating process.
428 THE EVOLUTION OF LIFE.
How in each, species, considered as an aggregate of indi- viduals, there must arise stronger and stronger contrasts between those divergent varieties which result from the instability of the homogeneous and the multiplication of effects, needs only be briefly indicated. It has already been shown {First Principles, § 126), that in conformity to the universal law that mixed units are segregated by lil^e incident forces, there are produced increasingly-definite distinctions among varieties, wherever there occur definitel}^- distinguished sets of conditions to which the varieties are re- spectively subject.
§ 157. Probably in tbe minds of some, the reading of this chapter has been accompanied by a running commentar}^, to the effect that the argument proves too much. The apparent implication is, that the passage from an indefinite, incohe- rent homogeneity to a definite, coherent heterogeneity in organic aggregates, must have been going on universally; whereas we find that in many cases there has been persist- ence without progression. This apparent implication, how- ever, is not a real one.
For though every environment on the Earth's surface undergoes changes; and though usually the organisms which each environment contains, cannot escape certain resulting new influences; yet occasionally such new in- fluences are escaped, by the survival of species in the un- changed parts of their habitats, or by their- spread into neighbouring habitats which the change has rendered like their original habitats, or by both. Any alteration in the temperature of a climate or its degree of humidity, is un- likely to affect simultaneously the whole area occupied by a species; and further, it can scarcely fail to hapj)en that the addition or subtraction of heat or moisture, will give to a part of some adjacent area, a climate like to that to which the species has been habituated. If, again, the circumstances of a species are modified by the intrusion of some foreign INTERNAL FACTORS. 429 kind of plant or animal, it follows that since the intruders will probably not spread throughout its whole habitat, the species will, in one or more localities, remain unaffected by them. Especially among marine ci'eatures, must there fre- quently occur cases in which modifying causes are con- tinually eluded. Much more uniform as are the physical conditions to which the sea exposes its inhabitants, it becomes possible for such of them as live on widely-diffused food, to be widely distributed; and wide distribution generally pre- vents the members of a species from being all subject to the same cause. Our commonest cirrhiped, for instance, subsisting on minute creatures that are everywhere dispersed through the sea; needing only to have some firm surface on which to build up its shell; and in scarcely any danger from sur- rounding animals; is able to exist on shores so widely remote from one another, that nearly every change in the actions of incident forces, must fall within narrower areas than that which the species occupies. In nearly every case, therefore, a portion of the species will survive unmodified. Its easily- transported germs will take possession of such new habitats as have been rendered fitter by the change that has unfitted some parts of its original habitat. Hence, on successive occasions, while some parts of the species are slightly trans- formed, another part may continually escape transformation by migrating hither and thither, where the simple condi- tions needed for its existence recur in nearly the same com- binations as before. And it will so become possible for it to survive, with comparatively trifling structural changes, throughout long geologic periods.
§ 158. The results to which we find ourselves led, are these.
In subordination to the different amounts and kinds of forces to which its diflerent parts are exposed, every in- dividual organic aggregate, like all other aggregates, tends to pass from its original indistinct simplicity towards a more 430 THE EVOLUTION OF LIFE.
distinct complexity. Unless we deny the persistence of force, we must admit that the gravitation of an organism's structure from an indefinitely homogeneous to a definitely heterogeneous state, must be cumulative in successive genera- tions, if the forces causing it continue to act. And for the lilie reasons, the increasing assemblage of individuals arising from a common stock, is also liable to lose its original uniformity; and, in successive generations, to grow more pronounced in its multiformity.
These changes, which would go on to but a comparatively small extent were organisms exposed to constant external conditions, are kept up by the continual changes in external conditions, produced by astronomic, geologic, meteorologic, and organic agencies: the average result being, that on previous complications of structure wrought by previous incident forces, new complications are continually superposed by new incident forces. And hence simultaneously arises increasing heterogeneity in the structures of individuals, in the structures of species, and in the structures of the Earth's Flora and Fauna.
But while, in very many or in most cases, the ever- changing incidence of forces is ever adding to the complexity of organisms, and to the complexity of the organic world as a whole; it does this only where its action cannot be eluded. And since, by migration, it is possible for species to keep themselves under conditions that are tolerably constant; there must be a proportion of cases in which greater hetero- geneity of structure is not produced.
Uniting these three propositions, we are brought to a con- clusion which, so far as it goes, appears to be in harmony with the facts. We find progression to result, not from a special, inherent tendency of living bodies, but from a general average efiect.of their relations to surrounding agencies. While wh are not called on to suppose that there exists in organisms any primordial impulse which makes them con- tinually unfold into more heterogeneous forms; we see that a liability to be unfolded arises from the actions and reactions between organisms and their fluctuating environ- ments. And we see that the existence of such a cause of development, presupposes the non-occurrence of development where this fluctuation of actions and reactions does not come into play.
To show, however, that there must arise a certain general tendency to the production of more heterogeneous aggregates, is not sufficient. It is quite conceivable that aggregates should be rendered more heterogeneous b}^ changing incident forces, without having given to them that peculiar form of heterogeneity required for carrying on the functions of life. Hence it remains now to inquire, how the production and maintenance of this peculiar form of heterogeneity is insured.
CHAPTER XI.
CHAPTER XI.
DIRECT EQUILIBRATION.
§ 159. Every cliange is of necessity towards a balance of forces; and of necessity can never cease until a balance of forces is reached. When treating of equilibration under its general aspects (First Principles, Part 11., Chap, xvi.), we saw that in every aggregate having compound movements, there tends continually to be established a moving equilibrium; since any unequilibrated force to which such an aggregate is subject, if not of a kind to overthrow the aggregate al- together, must continue modifying its state im.til an equi- librium is brought about. And we saw that the structure simultaneously reached must be " one presenting an arrange- ment of forces that counterbalance all the forces to which the aggregate is subject; " since, " so long as there remains a residual force in any direction — be it excess of a force exercised by the aggregate on its environment, or of a force exercised by its environment on the aggregate, equilibrium does not exist; and therefore the re- distribution of matter must continue."
It is essential that this truth should here be fully under- stood; and to the end of insuring a clear comprehension of it, some re-illustration is desirable. The case of the Solar System will best serve our purpose. An assemblage of bodies, each of which has its simple and compound motions, that severally alternate between two extremes, and the whole of DIRECT EQUILliniATION. 433 which has its involved perturbations, that now increase and now decrease, is here presented to us. Suppose a new force were brought to bear on this moving equilibrium — say- by the arrival of some wandering mass, or by an additional momentum given to one of the existing masses — what would be the result? If the strange body or the extra force were very large, it might so derange the entire system as to cause its collapse: by overthrow of its rhythmical movements, the moving equilibrium might rapidly be changed into a com- plete equilibrium. But what if the incident force, falling on the system from mthout, proved insufficient to overthrow it? There woidd then arise a set of perturbations which would, in the course of an enormous period, slowly work round into a modified moving equilibrium. The effects primarily im- pressed on the adjacent masses, and in a smaller degree on the remoter masses, would soon become complicated wdth the secondary effects impressed by the disturbed masses on one another; and these again with tertiary effects. Waves of perturbation woidd continue to be propagated throughout the entire system; until, around a new centre of gravity, there had been established a set of planetary motions more or less different from the preceding ones. All this would necessarily follow from the truths that any new force brought to bear on a moving equilibrium, must gradually be used up in overcoming the forces that resist the divergence it gener- ates: which antagonizing forces, being then no longer op- posed, set up a counter- action, ending in a compensating divergence in the opposite direction, that is followed by a re- compensating divergence; and so on, until there is either established some additional rhythmical movement, or some equivalent modification of the pre-existing rhythmical move- ments. N^ow though instead of being, like the Solar Sy^stem, in a state of independent moving equilibrium, an organism is in a state of dependent moving equilibrium {First Principles, § 130); yet this does not prevent the manifestation of the same law. Every animal daily obtains 23 434 THE EVOLUTION OF LIFE.
from without, a supply of force to replace the force which it expends; but this continual giving to its parts a new momentum, to make up for the momentum continually lost, does not interfere with the carrying on of actions and reactions like those just described. Here, as before, we have a definitely-arranged aggregate of parts, which we call organs, having their definitely-established actions and ^ re- actions, which we call functions. These rhythmical actions or functions, and the various compound rhythms resulting from their combinations, are in such adjustment as to balance the actions to which the organism is subject: there is a con- stant or periodic genesis of forces, which, in their kinds, amounts, and directions, suffice to antagonize the forces which the organism has constantly or periodically to bear. If then there exists this state of m.oving equilibrium among a definite set of internal actions, exposed to a definite set of ex- ternal actions; what must result if any of the external actions are changed? Of course there is no longer an equilibrium. Some force which the organism habitually generates, is too great or too small to balance some incident force; and there arises a residuary force exerted by the environment on the organism, or by the organism on the environment. '• This residuary force — this unbalanced force, of necessity expends itself in producing some change of state in the organism. Acting directly on some organ and modifying its function, it indirectly modifies dependent functions, and remotely influences all the functions. As we have already seen (§§ 68, 69), if this new force is permanent, its effects must be gradually diffused throughout the entire system; until it has come to be equilibrated in working those structural re- arrangements which produce an exactly counterbalancing force.
The bearing of this general truth on the question we are now dealing with, is obvious. Those modifications upon modifications, which the unceasing mutations of their en- vironments have been all along generating in organisms, DIRECT EQUILIBRATION. l'3-3 have been in each case modificatioiLS involved by the cstablisliment of a new balance with the new combination of conditions. In every species througliout all geologic time, there has been perpetually going on a rectification of the equilibrium, that has been perpetually disturbed by the alteration of surrounding circumstances; and every further heterogeneity has been the addition of a structural change entailed by a new equilibration, to the structural changes entailed by previous equilibrations. There can be no other ultimate interpretation of the matter, since change can have no other goal. Any fresh force brought to bear on an aggregate in a state of moving equilibrium, must do one of two things: it must either overthrow the moving equi- librium altogether, or it must alter without overthrowing it; and the alteration must end in the establishment of a new moving equilibrium. Hence in organisms, death or restora- tion of the physiological balance, are the only alternatives.
This equilibration between the functions of an organism and the actions in its environment, may be either direct or indirect. The new incident force may either immediately call forth some counteracting force, and its concomitant structural change; or it may be eventually balanced by some otherwise-produced change of function and structure. These two processes of equilibration are quite distinct, and must be separately dealt with. We will devote this chapter to the first of them.
§ 160. Direct equilibration is that process currently knoAVn as adaptation. We have already seen (Part II., Chap, v.), that individual organisms become modified when placed in new conditions of life — so modified as to re-adjust the powers to the requirements; and though there is great difficulty in disentangling t^e evidence, Ave found reason for thinking (§ 82) that structural changes thus caused by functional changes are inherited. In the last chapter, it was argued that if, instead of the succession of individuals 436 THE EVOLUTION OF LIFE, constituting a species, there were a continuously- existing individual, any such functional and structural divergence as we see produced by a new incident force, would necessarily go on increasing until the new incident force was counter- poised; and that the rejplacing of a continuously-existing individual by a succession of individuals, each formed out of the modified substance of its predecessor, will not prevent the like effect from being produced — the persistence of force negativing any other inference. Here we further find, that this limit towards which any such organic change advances, in the species as in the individual, is a new moving equi- librium adjusted to the new arrangement of external forces.
But now, what are the conditions under which alone, direct equilibration can occur? Are all the modifications that serve to re-fit organisms to their environments, directly adaptive modifications? And if otherwise, which are the directly adaptive and which are not? How are we to distinguish between them?
Manifestly^ for any moving equilibrium to be gradually altered, it is needftd, first, that some force shall operate upon it; and, second, that the force shall not be such as to over- throw it. If in the environment there exists some agency that would act advantageously on an organism were the or- ganism a little modified, but which does not act on it in the absence of the required modification; it is clear that this agency cannot itself tend to produce the modification. On the other hand, if the external agency be of such kind, that individuals of the species whenever affected by it, are either killed or so injured that the production of vigorous offspring- is much interfered with, there cannot be directly wrought in the species, any such alteration as will fit it to cope with this external agency. The only new incident forces which can work the changes of function and structure required to bring any animal or plant into equilibrium with them, are such incident forces as operate on this animal or plant, either continuously or frequently. They must be capable DIRECT EQUlIiTlJ RATION. 437 of appreciably clianging that set of complex rhythmical actions and reactions constituting the life of the organism; and yet must not usually produce perturbations that are fatal. Let us see what are the limits to direct equilibra- tion hence arising.