DARWINIAN THEORY 



Darwinian Theory* tho explanation of t In- 

 volution of lorms of life on our planet, oH'rri-d 

 by the great naturalist, Charles Darwin (q.v. ). 

 from the very outset we must carefully guard 

 against the confusion, still widely popular, of 

 1 K.\ illation ' with ' Darwinism.' Evolution must 

 clearly !> retained to denote the entire drama of 

 -h.-in-i- ; Darwinism, therefore, must as 

 y ! iv>tricted to one particular interpreta- 

 tion of the mechanism and plot of this cosmic 

 drama, of many which have been thrown out by 

 reflective spectators. Darwin expressed the mode 

 of the evolutionary process in a classic phrase, 

 the title of his great work The Origin of Species 

 by means of Natural Selection, or the Preservation 

 of Favoured Races in the Struggle for Life. That 

 explanation is not only in itself peculiarly shrewd 

 and well reasoned, luminous and widely appli- 

 cable, but has been of the very greatest import- 

 ance in awakening biologists and students of all 

 other sciences, aim thus ultimately the intelligent 

 public, to the facts of the case to the conscious- 

 ness of this evolutionary drama. On the pro- 

 mulgation of this new hypothesis as to the mode of 

 occurrence of evolutionary changes in the organic 

 world, the whole discussion as to the occurrence of 

 evolutionary change at all speedily came to centre 

 round it ; although in view of the powerful argu- 

 ments for the occurrence of evolution which had 

 been independently adduced by Spencer and others, 

 it may be admitted this was not quite logical. The 

 fact, however, remains. Moreover to the new stand- 

 point afforded by the clear acceptance of both ideas 

 evolution and this through natural selection and 

 to the energetic application of these, first by Darwin 

 himself, but soon by a multitude of zealous workers, 

 we owe a progress which it would as yet be prema- 

 ture to estimate, but which has pervaded the whole 

 field of biology, and even the fields of all the higher 

 sciences, mental and social, which so largely utilise 

 biological methods and generalisations. Whatever 

 may Ye the subsequent development of our evolu- 

 tionary conceptions, the epoch-making importance 

 of the Darwinian theory will be unaltered. Hence 

 the expediency of the present comparatively full 

 exposition of its main positions and of their bear- 

 ings, apart from the larger and more general argu- 

 ment under EVOLUTION. 



To the statement of Darwin's theory, therefore, 

 we may at once proceed, postulating no more than 

 that general acquaintance with the amis and results 

 of biology which is now Incoming so commonly 

 current, or which may readily be gained by help of 

 the articles BIOLOGY and BOTANY. Nor need any 

 statement of the general doctrine of evolution, or of 

 evolutionary theories before Darwin, here detain us, 

 since these find more fitting place in the general 

 article EVOLUTION. 



The failure of pre-Darwinian theories to gain 

 any very general acceptance among naturalists 

 was no doubt largely attributable to established 

 prejudice, backed as this was by the predominant 

 authority of Cuvier. This was by no means the 

 whole explanation. For while those theories 

 rendered it extremely probable that modification 

 had occurred, they all fell short, as Darwin pointed 

 out, in one most important particular. They 

 failed to show how the modification of one species 

 from another could take place, 'so as to acquire 

 that perfection of structure and co-adaptation 

 which justly excites our admiration ; ' since the 

 hypotheses of the potency of external conditions, 

 of habit, of the volition of the organism itself, and 

 so on, alike successively broke down. 



Darwin was especially struck by the distri- 

 butional phenomena he witnessed during his 

 'Naturalist's Voyage,' and thereafter devoted 

 himself primarily to the problem of the origin of 



species, specially concentrating hiniMdf upon what 

 seemed to him the weakest |M>int of tin- preceding 

 theories, the explanation of adaptations. Com 

 mencing in 1K.S7, 'after five years work, 1 allowed 

 myself to speculate on the subject, and drew up 

 some short notes ; these I enlarged in 1844 into a 

 sketch of the conclusions which then seemed to be 

 probable ; from that period to the present day I 

 nave steadily pursued the same object.' This was 

 written as late as 1859, and even then because he 

 had received a paper from Mr A. H. Wallace (at 

 that time exploring the Malay Archipelago), in 

 which views identical with his own were expressed, 

 and thus was compelled to proceed to the publica- 

 tion of his results. He did so iirst in a brief out- 

 line read, by advice of Lyell and Hooker, simul- 

 taneously with Wallace's pajxr (see Jour. Linn. 

 Soc., 1858); and in the following year in that fuller 

 abstract, as at first he merely regarded it, which 

 soon became so famous the Origin of Species. 

 Its substance may with advantage be briefly sum- 

 marised, so far as further compression of such 

 ' intellectual pemmican ' is possible. For details 

 and explanations the reader must consult the work 

 itself ( 7th ed. 1880). 



Outline of 'Origin of Species.' In order to ain 

 insight, then, into the means of modification, 

 Darwin commences with a study of the variation 

 of plants and animals under domestication (later 

 expanded into a separate work ; 2d ed. 1876). 



Variation and Heredity. While all plants and 

 animals exhibit some degree of variation, this is 

 greatest among domesticated species, owing to 

 their new and less uniform conditions of life. 

 These may act directly on the whole organisation, 

 or on separate parts, and the variation, though 

 rarely, is sometimes definite, as when size increases 

 with quantity of food, or colour changes with its 

 quality ; or the conditions may act indirectly by 

 influencing the reproductive system, which is pecu- 

 liarly sensitive. Changed habits produce an in- 

 herited effect e.g. the leg-bones of the common 

 duck weigh proportionally more, and its wing-l>ones 

 less, than in the wild variety, because it flies less 

 and walks more. So, too, tame mammals acquire 

 drooping ears, since these are rarely pricked in 

 alarm. One variation is usually correlated with 

 others, thus long-beaked pigeons have small feet, 

 and conversely. All variations tend to be in- 

 herited. The popular belief that domestic races 

 simply revert to the aboriginal stock is unsupported 

 by facts. 



Save that domestic varieties are less uniform 

 than wild species, often differ more widely in some 

 single part, and are fertile when crossed, there is 

 no well-marked distinction between these and so- 

 called true species. If, therefore, such varieties as 

 the different breeds of the dog can be shown to le 

 descended from a single wild species, there neces- 

 sarily arises great doubt as to the immutability of 

 closely al'ied natural species, such as the foxes. 

 While, however, the many breeds of dog appear to 

 have arisen from several wild species, and those of 

 cattle also from two or three, our fowls, ducks, 

 rabbits, &c., all certainly arise from a single ances- 

 tral species. The case of pigeons is of peculiar im- 

 Sortance, since pouter, carrier, fantail. and tumbler 

 iffer so thoroughly, externally and internally, that 

 any ornithologist would be compelled to assign to 

 them not merely specific but generic distinctness, 

 if he had discovered them in the wild state. There 

 is at least as much difficulty in believing that such 

 breeds can have proceeded from a common ancestor, 

 as there is in the case of any group of birds in 

 nature ; and every breeder of these has been firmly 

 convinced of their descent from distinct species. 

 Yet all these breeds are proved to come from the 

 common rock-dove (Lolumba livia; see PIGEON), 



