Evolutionary hypothesis

“ But, leaving special genesis aside, let us consider a little more closely the doctrine of natural selection. This, in fact, is not a theory of the origin—of the genesis of species. As M. Quatrefages has remarked, it is not a theory at all, for it explains nothing, accounts for nothing, and is not therefore truly an hypothesis in philosophy. Natural selection is not a cause, but the discussion of it draws attention to the chain of causes at the bottom of which we must look for the solution of our problem. ”

“ If the theory that the different species of animals now known to us have been evolved successively by descent from some primordial simplest form through modifications induced by change of habitation, of medium of life, and accumulation of new structures occurring through an immense period of time, be a sound hypothesis, the process which has evolved superior out of inferior organizations ought, in consistency with itself and with all its supposed conditions, to be capable of being reversed, so as to lead to the evolution of inferior out of superior organisms. ”

“ Let us recall what we have already established in regard to the evolutionary power of natural selection, namely, that the changes which it controls can never transcend the range of their utility, and it will be clear to us that, of the many ids which make up the germ-plasm of the species, only so many will be modified as are necessary to evoke the character which has varied. ”

“ Since the appearance of Darwin’s Origin of Species, and after the great naturalist had converted the world to a belief in the general doctrine of evolution, there has arisen in the minds of many working naturalists a conviction that natural selection, or Darwinism as such, is only one of other evolutionary factors ”

“ Following the safe example of the physicists, and acknowledging the fact of the diversification of a once homogeneous species into varieties, we may receive the theory of the evolution of these into species, even while for the present we hold the hypothesis of a further evolution in cool suspense or in grave suspicion. In respect to very many questions a wise man's mind rests long in a state neither of belief nor unbelief. ”

“ They agree in emphatically asserting that Darwin's hypothesis of the origination of species through variation and natural selection "repudiates the whole doctrine of final causes," and "all indication of design or purpose in the organic world . . . is neither more nor less than a formal denial of any agency beyond that of a blind chance in the developing or perfecting of the organs or instincts of created beings. ”

“ And herein consisted his great advance upon the earlier evolutionism of Lamarck, Goethe, and Erasmus Darwin. For while these instinctive pioneers of the evolutionary spirit saw clearly that animals and plants betrayed signs of common descent from one or a few original ancestors, they did not see what was the mechanism by which such organisms had been differentiated into so many distinct genera and species. They caught, indeed, at the analogy of variation under domestication and in the wild state, but they missed the subtler and deeper analogy between human and natural selection. ”

“ In my earliest criticisms of the 'Origin' I ventured to point out that its logical foundation was insecure so long as experiments in selective breeding had not produced varieties which were more or less infertile; and that insecurity remains up to the present time. But, with any and every critical doubt which my sceptical ingenuity could suggest, the Darwinian hypothesis remained incomparably more probable than the creation hypothesis. ”

“ When to uncertainties in the arguments supporting the hypothesis we add its inability to explain facts of cardinal significance, as proved above, there is I think ground for asserting that natural selection is less clearly shown to be a factor in the origination of species than is the inheritance of functionally-wrought changes. ”

“ Resemblances, therefore, in the convolutions of different parts of this organ, as found in different vertebrate animals, however close they may be, prove nothing more than the adoption of a general plan for the production of objects common to the whole class of the vertebrate animals; and unless we can find other and independent proof that one species was descended from another by connection of lives with lives through successive generations, the hypothesis of special creations of the different species is not excluded by the facts. ”

“ If we seek for the reason of the difference between the scientific position of the doctrine of evolution a century ago, and that which it occupies now, we shall find it in the great accumulation of facts, the several classes of which have been enumerated above, under the second to the eighth heads. For those which are grouped under the second to the seventh of these classes, respectively, have a clear significance on the hypothesis of evolution, while they are unintelligible if that hypothesis be denied. ”

“ At the same time it must be steadily remembered that there are many naturalists at the present day, especially among those of the lower order of intelligence, who, while accepting evolutionism in a general way, and therefore always describing themselves as Darwinians, do not believe, and often cannot even understand, the distinctive Darwinian addition to the evolutionary doctrine—namely, the principle of natural selection. ”

“ A vitalistic view of evolution is rapidly developing in the scientific world, and the weight of evidence is decidedly in favour of regarding all evolutionary processes, reaching from the lowest to the highest organisms, as illustrating a gradual unfolding in the sensuous world of a pre-existing psychical power through an ever-increasing complexity of specialized structures, this complexity being brought about by natural 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. ”

“ 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 possibility of the process of evolution, as a means of accounting for the existence of any known animal, depends in some degree upon the animals among which, by sexual generation, the supposed transition from one kind of animal to another kind has taken place. Darwin speaks of the difficulty of defining "species" ”

“ On the theory of the evolution of animal out of animal, we must suppose that at some time the secondary causes of natural and sexual selection have done their work. It ends in the production of a species which thereafter remains one and the same animal, and Nature has established a barrier to any sexual union with any other species. ”

“ It is now more than fifty years since Darwin gave the theory of natural selection to the world, and the conception of a gradual evolution has long ago become part of the currency of thought. Evolution for Darwin was brought about by more than one factor. ”

“ The conviction that no theory of the origin of species could be satisfactory which failed to offer an explanation of the way in which mechanisms involving adaptations of structure and function to the performance of certain operations are brought about, was, from the first, dominant in Darwin's mind. ”

“ Of all the mischievous misconceptions of Darwin’s theory none have worked so much harm as that which regards natural selection as the active and efficient cause of evolution. Although evolution is an established fact, our knowledge of its processes are incomplete and must always remain so until we have solved that most vexed of all biological problems, the “causes of variation.” ”

“ The evolutionary concept is to biology what the doctrine of the conservation of energy has been in the physico-chemical sciences—an indispensable preliminary to proper methods of attack. But while great stress has been laid on the various methods by [Pg 253] which evolution may be supposed to have taken place—natural selection, Lamarckism, orthogenesis and the rest—biology has concerned herself comparatively little with the form of the process in itself. But it is here that evolution becomes of value to us in our present search; for once more we become aware of a direction. ”

“ On the foundation of the previous chapters, I will henceforth assume that we all accept organic evolution as a fact, without requiring any of the accessory evidence which is gained by independent proof of natural selection as a method. ”

“ It seems singular, in view of the facts of palæontology, that evolutionists of the Darwinian school are so wedded to the idea of one introduction only of each form of life, and its subsequent division by variation into different species, as it progressively spreads itself over the globe, or is subjected to different external conditions. ”

“ The Darwinian doctrine of evolution depends essentially on THE CUMULATIVE AUGMENTATION of minute variations in the direction of utility. But can such minute variations, which are undoubtedly continually appearing among the individuals of the same species, possess any selection-value; can they determine which individuals are to survive, and which are to succumb; can they be increased by natural selection till they attain to the highest development of a purposive variation?
To many this seems so improbable that they have urged a theory of evolution by leaps from species to species. ”

“ But as it is scientific to argue from the known to the unknown, or from the better known to the less known; and as to know fully what a thing is we must know what it is capable of becoming or producing, let us pass from the pre-animal to the animal stage of evolution. It is the more necessary to do this because it was Darwin's theory of the origin [262] of species which impressed upon the modern mind the idea that Nature mimics purpose, having none. ”

“ This, indeed, might have been expected; for as natural selection acts through one form having some advantage over other forms in the struggle for existence, it will chiefly act on those which already have some advantage; and the largeness of any group shows that its species have inherited from a common ancestor some advantage in common. ”

“ It has often been asked, if natural selection be so potent, why has not this or that structure been gained by certain species, to which it would apparently have been advantageous? But it is unreasonable to expect a precise answer to such questions, considering our ignorance of the past history of each species, and of the conditions which at the present day determine its numbers and range. ”

“ Now things are wholly changed, and almost every naturalist admits the great principle of evolution. There are, however, some who still think that species have suddenly given birth, through quite unexplained means, to new and totally different forms: but, as I have attempted to show, weighty evidence can be opposed to the admission of great and abrupt modifications. ”

“ This enables us to avoid most of the disputed questions as to the development of animals, and to confine ourselves to those general principles regulating the origin and development of species and genera which were first laid down by Mr. Darwin thirty years ago, and have now come to be adopted by naturalists as established propositions in the theory of evolution. ”

“ These processes are real and natural, not imaginative or artificial, and so they will remain even though it will become clear that much is still to be learned about the causes of variation and the course of biological inheritance. Darwin was the first to contend that natural selection is but a part of nature's method of accomplishing evolution. ”

“ In view of these plain facts and their logical conclusion it is curious to notice how Darwin in his Origin of Species constantly mingles together arguments to prove the proposition that evolution has occurred, that the structure and relations of existing animals can only be explained by descent with modification, with arguments and evidence in favour of natural selection as the explanation and cause of evolution. ”