530 THE POPULAR SCIENCE MONTHLY 



turn to favorable circumstances would rapidly increase in numbers and occupy 

 the place of the extinct species and variety. 



The variety would now have replaced the species, of which it would be a more 



perfectly developed and more highly organized form Here, then, we have 



progression and continued divergence deduced from the general laws which regu- 

 late the existence of animals in a state of nature, and from the undisputed fact 

 that varieties do frequently occur. . . . Variations in unimportant parts might 

 also occur, having no perceptible effect on the life-preserving powers; and the 

 varieties so furnished might run a course parallel with the parent species, either 

 giving rise to further variations or returning to the former type. ... In the wild 

 animal, on the contrary, all its faculties and powers being brought into full ac- 

 tion for the necessities of existence, any increase becomes immediately available, 

 is strengthened by exercise, and must even slightly modify the food, the habits and 

 the whole economy of tlie race. It creates, as it were, a new animal, one of su- 

 perior powers, and whicli will necessarily increase in numbers and outlive those 

 inferior to it. . . . 



We see, tlieri, that no inferences as to varieties in a state of nature can be 

 deduced from the observation of those occurring among domestic animals. . . . 

 Domestic animals are abnormal, irregular, artificial; they are subject to varieties 

 which never occur and never can occur in a state of nature: their very existence 

 depends altogether on human care; so far are many of them removed from that 

 just proportion of faculties, that true balance of organization . . . will also 

 agree with the peculiar character of the modifications of form and structure which 

 obtain in organized beings — the many lines of divergence from a central type, 

 the increasing efficiency and power of a particular organ through a succession of 

 allied species, and the remarkable persistence of unimportant parts, such as color, 

 texture of plumage and hair, form of horns or crests, through a series of species 

 differing considerably in more essential characters. . . . This progression, by mi- 

 nute steps, in various directions, but always checked and balanced by the neces- 

 sary conditions, subject 'to which alone existence can be preserved, may, it is be- 

 lieved, be followed out so as to agree with all the phenomena. . . . 



It is true that Wallace subsequently modified his theory, adopted the 

 selection of plus and minus fluctuations, and became a determined 

 opponent of the mutation hyijothcsis of De Vries. 



The distinctive features of the later development of the theory in 

 Wallace's mind were his more implicit faith in selection, his insistence 

 on utility or selection value of new or varying characters, his flat rejec- 

 tion of Lamarckism, his reliance on spontaneous variations as supplying 

 all the materials for selection. This confidence appears in the following 

 passages from his militant reply in the volume of 1889 to the critics of 

 Darwinism : 



The right or favorable variations are so frequently present that the un- 

 erring power of natural selection never wants materials to work upon. . . . The 

 importance of natural selection as the one invariable and ever-present factor in 

 all organic change and that which can alone have produced the temporary fixity 

 combined with the secular modification of species. 



The principle of discontinuity is less clearly brought out than in the 

 first sketch of 1858 ; the selection of fluctuation is favorably considered. 

 The laws and causes of variation are, however, assumed rather than 



