On joining with recognition of this general process a recognition of the tendency towards localization of deposit, one possible origin of specific marks is suggested. When in an organism the circulating fluids contain useless matter, normal or abnormal, the excretion of it, once determined towards a certain place, continues at that place. Trees furnish examples in the casting out of gums and resins. Animal life yields evidence in gouty concretions and such morbid products as tubercle. A place of enfeebled nutrition is commonly chosen—not unfrequently a place where a local injury has occurred. Now if we extend this principle, well recognized in pathological processes, to physiological processes, we may infer that where an adaptive modification has so reacted on the blood as to leave some matter to be got rid of, the deposit of this, initiated at some place of least resistance, may produce a local structure which eventually becomes a species-mark. A relevant inquiry suggests itself—What proportion of species-marks are formed out of inanimate tissue or tissue of low vitality—tissue which, like hair, feathers, horns, teeth, is composed of by-products unfit for carrying on vital actions.
§ 174e. In the days when, not having been better instructed by Mr. Darwin, I believed that all changes of structure in organisms result from changes of function, I held that the cause of such changes of function is migration. Assuming as the antecedent of migration a great geologic change, such as upheaval of the East Indian Archipelago step by step into a continent, it was argued, in an essay I then wrote, that, subjected primarily to new influences in its original habitat, each kind of plant and animal would secondarily be subjected to the altered conditions consequent on spreading over the upheaved regions.
"Each species being distributed over an area of some extent, and tending continually to colonize the new area exposed, its different members would be subject to different sets of changes. Plants and animals spreading towards the equator would not be affected in the same way with others spreading from it. Those spreading towards the new shores would undergo changes unlike the changes undergone by those spreading into the mountains. Thus, each original race of organisms would become the root from which diverged several races differing more or less from it and from one another."
It was further argued that, beyond modifications caused by change of physical conditions and food, others would be caused by contact of the Flora and Fauna of each island with the Floras and Faunas of other islands: bringing experience of animals and plants before unknown.[[61]]
While this conception was wrong in so far as it ascribed the production of new species entirely to inheritance of functionally-wrought alterations (thus failing to recognize Natural Selection, which was not yet enunciated), it was right in so far as it ascribed organic changes to changes of conditions. And it was, I think, also right in so far as it implied that isolation is a condition precedent to such changes. Apparently it did not occur to me as needful to specify this isolation as making possible the differentiation of species; since it goes without saying that members of a species spreading east, west, north, south, and forming groups hundreds of miles apart, must, while breeding with those of the same group be prevented from breeding with those of other groups—prevented from having their locally-caused modifications mutually cancelled.
The importance of isolation has of late been emphasized by Dr. Romanes and others, who, to that isolation consequent on geographical diffusion, have added that isolation which results from difference of station in the same habitat, and also that due to differences in the breeding periods arising in members of the same species. Doubtless in whatever way effected, the isolation of a group subject to new conditions and in course of being changed, is requisite as a means to permanent differentiation. Doubtless also, as contended by Mr. Gulick and Dr. Romanes, there is a difference between the case in which an entire species being subject to the same conditions is throughout modified in character, thus illustrating what Mr. Gulick calls "monotypic evolution," and the case in which different parts of the species, leading different lives, will, if they are by any means prevented from inter-breeding with other parts, form divergent varieties: thus illustrating "polytypic evolution."
§ 174f. Beyond geographical and topographical isolation, there is an isolation of another kind regarded by some as having had an important share in organic evolution. Foreshadowed by Mr. Belt, subsequently enunciated by Mr. Catchpool, fully thought out by Mr. Gulick, and more recently elaborated by Dr. Romanes, "Physiological Selection" is held to account for the genesis of marked varieties side by side with their parents. It is contended that without the kind of isolation implied by it, variations will be swamped by inter-crossing, and divergence prevented; but that by the aid of this kind of isolation, a uniform species may be differentiated into two or more species, though its members continue to live in the same area.
Facts are assigned to show that slightly unlike varieties may become unable to inter-breed either with the parent-species or with one another. This mutual inferiority is not of the kind we might expect. We might reasonably suppose that when varieties had diverged widely, crossing would be impracticable, because their constitutions had become so far unlike as to form an unworkable mixture. But there seems evidence that the infertility arises long before such a cause could operate, and that instead of failure to produce a workable constitution, there is failure to produce any constitution at all—failure to fertilize. Some change in the sexual system is suggested as accounting for this. That a minute difference in the reproductive elements may suffice, plants prove by the fact that when two members of slightly-divergent varieties are fertilized by each other's pollen, the fertility is less than if each were fertilized by the pollen of its own variety; and where the two kinds of pollen are both used, that derived from members of the same variety is prepotent in its effect over that derived from members of the other variety.
The writers above named contend that variations must occur in the reproductive organs as well as in other organs; that such variations may produce relative infertility in particular directions; and that such relative infertility may be the first step towards prevention of crossing and establishment of isolation: so making possible the accumulation of such differences as mark off new species. Without doubt we have here a legitimate supposition and a legitimate inference. Necessarily there must happen variations of the kind alleged, and considering how sensitive the reproductive system is to occult influences (witness among ourselves the frequent infertility of healthy people while feeble unhealthy ones are fertile), it is reasonable to infer that minute and obscure alterations of this kind may make slightly-different varieties unable to inter-breed.
Granting that there goes on this "physiological selection," we must recognize it as one among the causes by which isolation is produced, and the differentiating influence of natural selection in the same locality made possible.