No. 579] PROGRESSIVE EVOLUTION 



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by supposing that their ancestors had well-developed 

 wings, by the aid of which they made their way to the 

 islands from some continental area. The conditions of 

 the new environment led to the gradual disuse and con- 

 sequent degeneration of the wings until they either be- 

 came useless for flight or, in the case of the moas, com- 

 pletely disappeared. It would be absurd to maintain 

 that any of the existing flightless birds are specifically 

 identical with the ancestral flying forms from which 

 they are descended, and it would, it appears to me, be 

 equally absurd to suppose that the flightless species arose 

 by mutation or by crossing, the same result being pro- 

 duced over and over again on different islands and in 

 different groups of birds. This is clearly a case where 

 the environment has determined the direction of evo- 

 lution. 



In such cases there is not the slightest ground for be- 

 lieving that crossing has had anything whatever to do 

 with the origin of the different groups to which the term 

 species is applied ; indeed, the study of island faunas in 

 general indicates very clearly that the prevention of 

 crossing, by isolation, has been one of the chief factors 

 in the divergence of lines of descent and the consequent 

 multiplication of species, and Eomanes clearly showed 

 that even within the same geographical area an identical 

 result may be produced by mutual sterility, which is the 

 cause, rather than the result, of specific distinction. 



Species, then, may clearly arise by divergent evolu- 

 tion under changing conditions of the environment, and 

 may become separated from one another by the extinc- 

 tion of intermediate forms. The environmental stimuli 

 (including, of course, the body as part of its own en- 

 vironment) may, however, act in two different ways: 

 (1) Upon the body itself, at any stage of its development, 

 tending to cause adaptation by individual selection of the 

 most appropriate response; and (2) upon the germ- 

 plasm, causing mutations or sudden changes, sports, in 



