Evolutionary descent

“ Passing from these difficulties, the other great leading facts in palæontology agree admirably with the theory of descent with modification through variation and natural selection. We can thus understand how it is that new species come in slowly and successively; how species of different classes do not necessarily change together, or at the same rate, or in the same degree; yet in the long run that all undergo modification to some extent. ”

“ Yet he looked upon the natural selection of small favourable variations as the principal factor in evolutionary change. Since Darwin's time the trend has been to magnify natural selection at the expense of the other two factors. ”

“ Darwin appeared to restrict his system to species, very soon, as was but natural, it was extended to the production of new genera, and even of divisions of the organic kingdoms yet wider asunder. Thus—apart from the most famous instance of all, treated by Darwin himself in his Descent of Man—it is now a cardinal point with Evolutionists generally that all the higher forms of life are descended from the lowest, and that even far up the line of development, creatures apparently the most diverse have sprung from one identical ancestor. ”

“ In most higher animals progress is brought about chiefly by natural selection operating upon individuals, although in a few forms selection operates chiefly upon groups of communities: in both cases the changes in the inherited constitution of the species are the important changes. In man, however, in all except the very early stages of his development, changes in inherited constitution have been small and unimportant, and the chief changes of evolutionary significance have been those in tradition ”

“ The comparison of artificial and natural selection has furnished material support for the theory of descent, and in turn been the object of constant criticism since the time of Darwin. The criticisms, in greater part, have arisen chiefly from an imperfect knowledge of both processes. By the aid of distinctions recently made possible, the contrast between elementary species and improved races has become much more vivid, and promises to yield better results on which to base comparisons of artificial and natural selection. ”

“ Natural selection, also, leads to divergence of character; for more living beings can be supported on the same area the more they diverge in structure, habits, and constitution, of which we see proof by looking to the inhabitants of any small spot or to naturalised productions. Therefore during the modification of the descendants of any one species, and during the incessant struggle of all species to increase in numbers, the more diversified these descendants become, the better will be their chance of succeeding in the battle for life. ”

“ From all this it is evident that the problem of the origin of species is as yet far from being solved. Biologists are now busily engaged in pushing their enquiries further back than was done by Darwin. Variations must be present for natural selection to work upon, so there is something to explain before natural selection can begin its work. ”

“ But if he vary away from the ancestral type, it does not necessarily follow that the variation will constitute an extension of the previous evolution. It may constitute a reversal of it, or a divergence in an altogether new direction, and, therefore, in the absence of selection, the variation of the offspring from the parent must tend on the whole to bring about retrogression—a tendency which is checked and reversed in an evolving species only by a sufficiently severe process of selection. ”

“ In the evolutionary theories of modern biology, it is precisely the problems of the origination of new species, and of the special character of the relations between the species and its environment by which this process is conditioned, that have attracted almost exclusive attention. And, with the steadily increasing success of evolutionary hypotheses in dealing with biological problems, there has naturally arisen a tendency to extend the application of the general concept of evolution far beyond the sphere in which it first originated. ”

“ On the other hand, it would be proof presumptive of natural selection if it could be shown that one species becomes slowly transmuted into another—i.e., that one set of adaptations may be gradually transformed into another set of adaptations according as changing circumstances require. ”

“ Those naturalists, on the other hand, who admit the principle of evolution, and this is now admitted by the greater number of rising men, will feel no doubt that all the races of man are descended from a single primitive stock; whether or not they think fit to designate them as distinct species, for the sake of expressing their amount of difference.301 With our domestic animals the question whether the various races have arisen from one or more species is different. ”

“ And the apparent existence of such a radical distinction between the varieties produced under domestication and true physiological species was an objection, not only against natural selection, but also against evolution itself; for it meant that we do not see now, nor within the limits of human observation, organisms actually getting transformed, through the accumulation of variations, into new species differing from their progenitors by the final test of specificity of character. ”

“ As I have said in the seventh essay, the fact of evolution is to my mind sufficiently evidenced by palaeontology; and I remain of the opinion expressed in the second, that until selective breeding is definitely proved to give rise to varieties infertile with one another, the logical foundation of the theory of natural selection is incomplete. We still remain very much in the dark about the causes of variation; the apparent inheritance of acquired characters in some cases; and the struggle for existence within the organism, which probably lies at the bottom of both of these phenomena. ”

“ Under the influence of natural conditions of which we have no exact knowledge, though traces of them are discoverable, species have been modified in a thousand ways: races have arisen which, becoming fixed, have thus produced a corresponding number of secondary species. ”

“ I am inclined to believe that almost every species (as we see with nearly all our domestic productions) varies sufficiently for Natural Selection to pick out and accumulate new specific differences, under new organic and inorganic conditions of life, whenever a place is open in the polity of nature. But looking to a long lapse of time and to the whole world, or to large parts of the world, I believe only one or a few species of each large genus ultimately becomes victorious, and leaves modified descendants. ”

“ In short, there is hardly any obvious character that has not, in one or other of the breeds, undergone great modification. As Darwin remarked, any naturalist coming upon such a group of forms in nature would have no hesitation in placing them in different species or genera, or even perhaps in different families. Even granting that the conditions of domestication are peculiar, we must admit that if such large changes can occur in a few centuries, it is possible that man has evolved from the lowest of living organisms during a period some hundreds of thousands of times as long. ”

“ The new species arising from natural selection maintain themselves much more permanently, and return much less easily to the original form, than is the case with products of artificial selection, and accordingly maintain themselves during a much longer time than the artificial races produced by man. ”

“ This natural selection will tend to produce divergence of character among offspring, and to intensify differences until they equal those between species or even genera. The same tendency to improvement brings about the decay and ultimate extinction of many lower and unimproved forms of life.
One of the best examples of Darwin’s style is in the passage comparing all members of the same class of beings to a great tree. ”

“ The most important condition of all is, undoubtedly, that variations should occur of sufficient amount, of a sufficiently diverse character, and in a large number of individuals, so as to afford ample materials for natural selection to act upon; and this, we have seen, does occur in most, if not in all, large, wide-ranging, and dominant species. From some of these, therefore, the new species adapted to the changed conditions would usually be derived ”

“ Darwin, then, himself and his most enthusiastic adherents must admit that at some given moment these races became suddenly incapable of crossing with their predecessors. Whence then arises the sterility which separates species? When, and at what moment will the physiological bond be broken, which unites the original species with its modified descendants, even when this modification is carried as far as the ordinary ox and the niata? What will be the determining cause of this great fact which obtains through the whole economy of the organic kingdom? ”

“ That species have a capacity for change will be admitted by all evolutionists; but there is no need, as it seems to me, to invoke any internal force beyond the tendency to ordinary variability, which through the aid of selection, by man has given rise to many well-adapted domestic races, and which, through the aid of natural selection, would equally well give rise by graduated steps to natural races or species. ”

“ Darwin firmly connected Variation to the Origin by devoting its introduction to an overview of his theory of evolution by natural selection. In particular, the two volumes of Variation, cumbersomely organized and packed with zoological and botanical detail, addressed some of the difficulties inherent in the attractive but paradoxical analogy between natural selection and artificial selection. For selection of any sort to operate, diversity already had to exist. With wild populations living under natural conditions, however, diversity was difficult to discern. ”

“ Darwin gives reasons for believing that when variation once begins it continues with some vigour; hence, between two settled wide-spread species connected genealogically together we might expect a large number of transitional varieties, each represented by only a few individuals, so that the whole number of these transitional forms might well be lost to the genealogical record. ”