George John Romanes

“ But the distinguishing peculiarity of natural selection, considered as a form of isolation, is that it effects the isolation by killing off all the individuals which it fails to isolate: consequently, this form of isolation differs from other forms in prohibiting the possibility of any ramification of a single group of intergenerants into two or more groups, for the purpose of carrying on the evolution in divergent lines. Therefore, under this form of isolation alone, evolution must proceed, palm-like, in a single line of growth. ”

“ But we may well hope that before another ten years shall have passed, even these still outstanding questions will have been finally settled; and thus that within the limits of an ordinary lifetime the theory of organic evolution will have been founded and completed in all its parts, to stand for ever in the world of men as at once the greatest achievement in the history of science, and the most splendid monument of the nineteenth century. ”

“ Had Darwin endeavoured to support his hypothesis by facts, he would, at least in the vegetable kingdom, have found little to favour his cause. I can cite many hundreds of cases, in which species in every stage of development have been found closely mingling with one another, and not in any way isolated. Therefore, I do not think that one can rightly speak of natural selection in the Darwinian sense in the vegetable kingdom; and, in my estimation, there is a great difference between the formation of species by nature and the production of stock by a breeder.... ”

“ Wallace, in his Darwinism, has done good service by enabling all other naturalists clearly to perceive how natural selection alone produces monotypic evolution—namely, through the free intercrossing of all individuals which have not been eliminated by the isolating process of natural selection itself. For he very lucidly shows how the law of averages must always ensure that in respect of any given specific character, half the individuals living at the same time and place will present the character above, and half below its mean in the population as a whole. ”

“ Yet the fact that hybrids are so comparatively rare in a state of nature—a fact which Sir Joseph Hooker has pointed out to me as otherwise inexplicable—proves the efficacy of even a low degree of such differentiation in preventing the physiologically-differentiated forms from intercrossing. Even in cases where there is no difficulty in producing artificial hybrids or mongrels between species or varieties growing on common areas, it is perfectly astonishing what an extremely small percentage of the hybrid or mongrel forms are found to occur in nature. ”

“ In particular, I have shown that segregation of the fit entirely relieves survival of the fittest from the difficulty under which it has hitherto laboured of explaining why it is that sterility is so constantly found between species, while so rarely found between varieties which differ from one another even more than many species; why so many features of specific distinction are useless to the species presenting them; and why it is that incipient varieties are not obliterated by intercrossing with parent forms. ”

“ That, in advancing the theory of physiological selection as "an additional suggestion on the origin of species," I wish to represent it as being the originating cause of all species. What I hold is, that all species must have owed their origin to isolation, in some form or other; but that as physiological selection is only one among many other forms of isolation (including natural selection) , and as it can only act on common areas, a large number of species must have been formed without its aid. ”

“ Nor can I believe, as Darwin incidentally and provisionally suggested, that prolonged exposure to uniform conditions of life have so generally induced an equally meaningless result. I can only believe that all the closely allied species inhabiting our supposed continent, and differing from one another in so many and such divers points of small detail, are merely so many records of the fact that selective fertility has arisen among their ancestry, and has thus given as many opportunities for the occurrence of morphological differentiations as it has furnished cases of efficient isolation. ”

“ For I take them to indicate a continued failure to perceive the all-important distinction between evolution as monotypic and polytypic. Unless one has fully grasped this distinction, and constantly holds it in mind, he is not in a position to understand the "difficulty" in question; nor can he avoid playing fast and loose with natural selection as possibly the sole cause of evolution, and as necessarily requiring the co-operation of some other cause. ”

“ Even if it were true that all species and all specific characters must necessarily owe their origin to natural selection, it would still remain illogical to define the theory of natural selection as indifferently a theory of species or a theory of adaptations ”

“ The reason why it is so easy to attack your theory is that it is so easy to confuse the survival of an individual with the [Pg 160] survival of a peculiarity of type. No one has ever said that an individual is assisted by the possession of selective fertility: that is a matter which cannot affect his chance of life. Nor has any one said that the possession of selective fertility in an individual will of itself increase the chance of his having progeny that will survive, and in turn become the progenitors of others that will survive. ”

“ Gulick says, "Natural selection is the exclusive breeding of those better adapted to the environment: ... it is a process in which the fittest are prevented from crossing with the less fitted, by the exclusion of the less fitted." Therefore it is, strictly and accurately, a mode of isolation, where the isolation has reference to adaptation, and is secured in the most effectual of possible ways—i.e. by the destruction of all individuals whose intercrossing would interfere with the isolation. ”

“ Any theory of the origin of species in the way of descent must be prepared with an answer to the question, Why have species multiplied? How is it that, in the course of evolution, species have not simply become transmuted in linear series instead of ramifying into branches? ”

“ I am convinced that the theory of physiological selection is the only one that can explain the facts of inter-specific sterility on the one hand, and, on the other hand, the contrast which these facts display to the unimpaired fertility of our domesticated varieties. ”

“ The importance of isolation as against dissimilar forms has always been fully appreciated by breeders, fanciers, horticulturists, &c., who are therefore most careful to prevent their pedigree productions from intercrossing with any other stock. Isolation is indeed, as Darwin has observed, "the corner-stone of the breeder's art." ”

“ The formation of a real variety, which Darwin, as we know, regards as the commencement of a new species, will only succeed when a few individuals, having crossed the barrier of their habitat, are able to separate themselves for a long time from the old stock." And the last sentence, given as a summary of his whole doctrine, is—"The geographical isolation of the form, a necessary consequence of migration, is the cause of its typical character." ”

“ If it be said that survival of the fittest may develop simultaneously two or more lines of useful change, the answer is that it can only do this if each of the developing varieties is isolated from the others by some additional form of isolation; for, if not, there can be no commencement of utilitarian divergence, since whatever number of utilitarian changes may be in course of simultaneous development, they must in this case be all blended together in a single line of specific transmutation. ”

“ Again, in his important essay on The Influence of Isolation, Weismann concludes, on the basis of a large accumulation of facts, that the constancy of any given specific type "does not arise suddenly, but gradually, and is established by the promiscuous intercrossing of all individuals." From which, he says, it follows, that this constancy must cease so soon as the condition which maintains it ceases—i. e. so soon as intercrossing (Panmixia) between all individuals ceases, or so soon as a portion of a species is isolated from its parent stock. To this principle he assigns the name of Amixia. ”

“ It is perfectly obvious that where the improvement of type in a linear series is concerned (monotypic evolution) , free intercrossing, far from being a hindrance to the process, is the very means by which the process is accomplished. Improvement here ascends by successive steps, in successive generations, simply because of the general intercrossing of the generally most fit with the result that the species, as a whole, gradually becomes transformed into another species, as a whole. ”

“ Hence it is that, while no Darwinian has ever questioned the power of unaided selection to cause improvement of character in successive generations, in common now with not a few other Darwinians I have emphatically denied so much as the abstract possibility of selection alone causing a divergence of character in two or more simultaneous lines of change. ”

“ There are doubtless many other causes of the origin of varieties besides cross-infertility with parent forms; but, as a general rule, it does not appear that they are by themselves capable of carrying divergence beyond a merely varietal stage. In order to carry divergence to the stage of producing species, it appears to be a general condition that, sooner or later, cross-infertility should arise—seeing that, when varieties do succeed in becoming species, we almost invariably find that, as a matter of fact, cross-infertility has arisen. ”

“ If we say by natural selection alone, we should expect to find the multitudinous species differing from one another in respect of features presenting well-marked adaptive meanings; yet this is precisely what we do not find. For Darwin's argument here is that "in large genera the amount of difference between the species is often exceedingly small, so that in this respect the species of the larger genera resemble varieties more than do the species of the smaller genera." ”

“ Allen's measurements have to do with—the law of averages will of course determine that half the whole number of variations in any given structure, in any given generation, will be above the mean line. But, equally of course, no one has ever denied that where, for either of these reasons, natural selection is provided with sufficient material, it is correspondingly capable of improving the specific type without the assistance of any other form of homogamy; so to speak, they protect themselves by their very numbers, and their superiority over others leads to their survival and accumulation. ”

“ But if in the course of many such abortive attempts, as it were, to produce a new species, nature happens to hit upon a structural or a colour variation which is capable of reacting on the sexual system in the particular way required, then this variation will be enabled to protect itself against free intercrossing in proportion to its own development. Or, in other words, the more it develops as a morphological change, the more will it increase the physiological change; while the more the physiological change is thus increased, the more will it in turn promote the morphological. ”

“ Even if such a thing could be imagined as happening occasionally, I feel it difficult to imagine that it can happen habitually, and yet this view must be held by those who would attribute prepotency to natural selection.
It must never be forgotten that the relatively enormous changes as to size, structure, habit, &c., which are presented by our domesticated plants as results of artificial selection, do not entail the physiological character of cross-sterility in any degree, save possibly in some small number of cases. ”

“ On the one hand, without the assistance of physiological selection, natural selection would, I believe, be all but overcome by the adverse influences of free intercrossing—influences all the more potent under the very conditions which are required for the multiplication of species by divergence of character. ”

“ Whether such a separation be due to geographical barriers, to migration, or to any other state of matters leading to exclusive breeding within the separated group, I shall indifferently employ the term isolation for the purpose of designating what in all cases is the same result—namely, a prevention of intercrossing between A and B, where A is the separated portion and B the rest of the species or kind. ”