Herbert Spencer

“ Those expansive and repressive energies which set to each species a limit that perpetually oscillates from side to side of a certain mean, are, as we lately saw, frequently changed by new combinations of the external factors—astronomic, geologic, meteorologic, and organic. Hence there from time to time arise lines of diminished resistance, along which the species flows into new localities. Such portions of the species as thus migrate, are subject to circumstances unlike its previous average circumstances. And from multiformity of the circumstances, must come multiformity of the species. ”

“ Each further complexity 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 ”

“ Given a series of organisms, each of which is developed from a portion of a preceding organism, and the question is whether, after exposure of the series for a million years to changed incident forces, one of its members will be the same as though the incident forces had only just changed. To say that it will, is implicitly to deny the persistence of force. In relation to any cause of divergence, the whole series of such organisms may be considered as fused together into a continuously-existing organism ”

“ 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. ”

“ Nearly always, 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 transformed, another part may continually escape transformation by migrating hither and thither, where the simple conditions needed for its existence recur in nearly the same combinations as before. ”

“ If, again, the circumstances of a species are modified by the intrusion of some foreign 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. ”

“ That tendency towards greater heterogeneity which is given to an organism by disturbing its environment, is helped by the tendency which every modification has to produce other modifications—modifications which must become more numerous in proportion as the organism becomes more complex. Lastly, among the indirect and involved manifestations of this tendency, we must not omit the innumerable small irregularities of structure which result from the crossing of dissimilarly-modified individuals. ”

“ Every differentiation of structure being a result of some difference in the relations of the parts to the agencies acting on them, it follows that the more multiplied and more unlike the agencies, the more varied must be the differentiations wrought. Hence the change from a state of homogeneity to a state of heterogeneity, will be marked in proportion as the environing actions to which the organism is supposes it is only are complex. ”

“ While it follows that every organism, considered as an individual and as one of a series, tends thus to pass into a more heterogeneous state; it also follows that every species, considered as an aggregate of individuals, tends to do the like. Throughout the area it inhabits, the conditions can never be absolutely uniform: its members must, in different parts of the area, be exposed to different sets of incident forces. ”

“ But now if any primordial organic aggregate must, in itself and through its descendants, gravitate from uniformity to multiformity, in obedience to the more or less multiform forces acting on it; what must happen if these multiform forces are themselves undergoing slow variations and complications? ”

“ Doubtless the condition under which alone increasing definiteness of structure can be acquired by any part of an organism, either in an individual or in successive generations, is that such part shall be exposed to some set of tolerably-constant forces ”

“ Any alteration in the temperature of a climate or its degree of humidity, is unlikely to affect simultaneously the whole area occupied by a species; and further, it can scarcely fail to happen that the addition or subtraction of heat or moisture, will give to a part of some adjacent area, a climate like that to which the species has been habituated. ”

“ In subordination to the different amounts and kinds of forces to which its different parts are exposed, every individual organic aggregate, like all other aggregates, tends to pass from its original indistinct simplicity towards a more distinct complexity. ”

“ That the minor sexual peculiarities are brought about by the physiological 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 development 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 many secondary changes, as well as tertiary changes still more multiplied. ”

“ Our commonest cirriped, for instance, subsisting on minute creatures everywhere dispersed through the water; needing only to have some firm surface on which to build up its shell; and in scarcely any danger from surrounding animals; is able to exist on shores so widely remote from one another, that nearly every change in the incident forces must fall within narrower areas than that which the species occupies. ”

“ How in each species, considered as an aggregate of individuals, there must arise stronger and stronger contrasts among those divergent varieties which result from the instability of the homogeneous and the multiplication of effects, need only be briefly indicated. ”

“ Comparatively 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 prevents the members of a species from being all subject to the same cause. ”

“ But a substance which is beyond all others changeable by the actions and reactions of the forces liberated from instant to instant within its own mass, must be a substance which is beyond all others changeable by the forces acting on it from without. ”

“ As interpreted in connexion with the ever-progressing, ever-complicating changes in external factors, it involves the conclusion that there is a prevailing tendency towards greater heterogeneity in all kinds of organisms, considered both individually and in successive generations; as well as in each assemblage of organisms constituting a species ”

“ This loss of homogeneity which the special instability of organic aggregates fits them to display more promptly and variously than any other aggregates, must be shown in more numerous ways in proportion as the incident forces are more numerous. ”

“ The vascular apparatus is modified throughout its whole structure by each considerable modification in the proportions of the body. Increase in the size of any organ implies a quantitative, and often a qualitative, reaction on the blood ”

“ That is, it must remain under influences calculated to make greater the definiteness of the chief differentiations already produced. If, for example, 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 arrangements have become thereby so far determined, that subsequent modifications are extremely likely, if not certain, to be consistent with the production of movement by the actions of muscles on a flexible central axis. ”

“ And since unlike forces must produce unlike results, each of these differentiated forces must produce, throughout the aggregate, a further series of differentiations." To this it was added that, in proportion as the heterogeneity increases, the complications arising from this multiplication of effects grow more marked; because the more strongly contrasted the parts of an aggregate become, the more different must be their reactions on incident forces, and the more unlike must be the secondary effects which these initiate ”

“ In any aggregate "the relations of outside and inside, and of comparative nearness to neighbouring sources of influence, imply the reception of influences that are unlike in quantity, or quality, or both ”

“ 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 see how profoundly a heavier head affects the entire osseous and muscular systems. ”