Evolutionary theory

“ Nor, for this purpose, have I to do with any theory of the "Origin of Species," much as I value that which is known as the Darwinian theory. That the doctrine of natural selection presupposes evolution is quite true; but it is not true that evolution necessarily implies natural selection. In fact, evolution might conceivably have taken place, without the development of groups possessing the characters of species. ”

“ 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. ”

“ Let us turn to define Evolution—a word about which much ambiguity exists. In its narrower sense 'Evolution,' a term not at first used by Darwin, means the morphological and physiological variations of organic beings by the action of natural selection. It then came to be loosely applied to almost any specific theory about the origin and development of things. ”

“ 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 ”

“ 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. ”

“ 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. ”

“ Darwin, the variations whose accumulation resulted in diversity of species and genus were indefinite, fortuitous, attributable but in small degree to any known causes, and without a general principle underlying them which would cause them to appear steadily in a given direction for many successive generations and in a considerable number of individuals at the same time. We did not know that the theory of evolution was one that had been quietly but steadily gaining ground during the last hundred years. ”

“ So the reasons for and against the evolution theory almost balance one another; and it is not improbable that the hypothesis of an origin of species through development will have to share its authority with the hypothesis of a descent of species through heterogenetic generation, as well as with the hypothesis of a primitive generation of lower organisms, still repeating itself at a later time. ”

“ 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. ”

“ 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 three illustrations now given sufficiently prove that, whenever the geological record approaches to completeness, we have evidence of the progressive change of species in definite directions, and from less developed to more developed types—exactly such a change as we may expect to find if the evolution theory be the true one. ”

“ 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 ”

“ Lastly, looking not to any one time, but to all time, if my theory be true, numberless intermediate varieties, linking most closely all the species of the same group together, must assuredly have existed; but the very process of natural selection constantly tends, as has been so often remarked, to exterminate the parent forms and the intermediate links. ”

“ Thus the improvement and modification induced by natural selection would be enhanced and transmitted by the sexual selection. [24]
In regard to the operation of the two kinds of selection in the evolution of man from a lower form of animal, we find the theory to be this: That organic beings with peculiar habits and structure have passed through transitions which have converted the primordial animal into one of totally different habits and structure; that, in these transitions, organs adapted to one condition and mode of life have become adapted to another ”

“ 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. ”

“ As according to the theory of natural selection an interminable number of intermediate forms must have existed, linking together all the species in each group by gradations as fine as are our existing varieties, it may be asked: Why do we not see these linking forms all around us? Why are not all organic beings blended together in an inextricable chaos? ”

“ 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 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 our theory the continued existence of lowly organisms offers no difficulty; for natural selection, or the survival of the fittest, does not necessarily include progressive development—it only takes advantage of such variations as arise and are beneficial to each creature under its complex relations of life. ”

“ 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. ”

“ 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. ”

“ 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" ”

“ The general principle of natural selection will account for this in the rough, maintaining as it does that fresh varieties, species, and genera better adapted to the surrounding circumstances have arisen, and by their superior adaptation unavoidably ousted the older forms. ”

“ 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. ”