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THE AMERICAN NATURALIST [Vol. LI 



to environment, (2) mutations occurring more or less in- 

 dependently of the environment and not necessarily of 

 adaptive value, (3) orthogenesis, whatever that may im- 

 ply. These are by no means mutually exclusive, and we 

 can see no reason why all, and others as well which we 

 have not time to consider, should not have been at work 

 producing the result we call organic evolution. 



It is not difficult to find, particularly among birds and 

 mammals, instances of specific variation giving rise to 

 local geographic races which are apparently the result of 

 response on the part of the organism to local conditions. 

 Again, it is easy, particularly among plants, to find vari- 

 eties, species and even genera which have arisen appar- 

 ently through sudden mutations and without anything 

 in the nature of adaptational response. Finally, paleon- 

 tology teems with apparent cases of orthogenetic 

 phylogeny which are not at present clearly explainable 

 in terms of natural selection or mutations. Before ex- 

 amining concrete cases which come under each of these 

 categories let us diverge for a moment to consider the 

 effect which natviral selection as a theory has had upon 

 biological conceptions. 



We see at once that the philosophical conception of con- 

 tinuity took an extraordinary hold on the minds of biol- 

 ogists. Largely as a result of the great influence of Dar- 

 win, towards the end of the nineteenth century continuity 

 in the origin of species became almost a fetish, and all 

 efforts were directed to showing how every character 

 whatsoever might have originated through the selection 

 of a series of gradually intergrading infinitesimal steps. 

 Yet it is more than doubtful if Darwin himself would 

 ever have been led into such an extreme position. Biolog- 

 ical philosophy has thus been ridden with the conception 

 that if a character could be shown to have arisen in a 

 gradual, piecemeal fashion its origin was thereby ex- 

 plained or accounted for, even though natural selection 

 could not be shown to have operated in its development. 

 On the other hand, the appearance of a character sud- 



