D.— ZOOLOGY 87 



accompanying the gradual process of character- divergence. In other 

 cases, such as Drosophila simulans, the two groups must be regarded as 

 species from the outset, although they may be indistinguishable in any 

 character save that which isolates them. At the opposite extreme are 

 those cases in which the factor inducing isolation simultaneously produces 

 considerable character-difference. This is so in Spartina Townsendi and 

 most cases of convergent and reticulate species-formation. Further 

 character-divergence may of course occur later. 



From the standpoint of natural selection, species will then fall into 

 two contrasted categories. On the one hand we have those in which 

 natural selection can have had nothing to do with the origin of the basic 

 specific characters, but merely acts upon the species as given, in competition 

 with its relatives. These include all species in which character-diver- 

 gence is abrupt and initial. On the other hand we have those forms in 

 which character-modification is gradual. Here natural selection may, 

 and on both deductive and inductive grounds often must, play a part in 

 producing the characters of the species. This helps to bring home the 

 heterogeneity of the processes which we lump together as ' evolution.' 



Adaptation and Selection. 



We next come to the origin of adaptations. It has been for some years 

 the fashion to decry the study or even to deny the fact of adaptation. I 

 have not the space to discuss the anti-adaptational attitude ; I will only 

 say that I believe it to be a passing fashion, and that, both structurally 

 and functionally, every organism is a bundle of adaptations, more or less 

 efficient, co-ordinated in greater or lesser degree. 



How has adaptation been brought about ? To-day biology rules out 

 special creation or divine guidance, frowns on entelechies and purposive 

 vital urges, and repudiates Lamarckism. 



Most biologists also look askance at orthogenesis setisu stricto, as im- 

 plying the inevitable grinding out of results predetermined by some 

 internal germinal clockwork. As Fisher has cogently pointed out, the 

 implications both of Lamarckism and of orthogenesis run directly counter 

 to the observed fact that the great majority of mutations are deleterious. 



There remains natural selection. Before discussing some concrete 

 examples of selection at work to produce adaptation and of adaptations 

 illustrating the work of natural selection, a few general points deserve to 

 be made. In the first place, there is the aged yet perennial fallacy that 

 such-and-such an arrangement cannot be adaptive, since related organisms 

 can and do exist without it. This is, quite frankly, nonsense. It is on 

 a par with saying that electric refrigerators are not useful because many 

 people manage to get on happily without them. 



There are numerous possible explanations of such a state of affairs. 

 It may be that mutations in that direction did not crop up, or were not 

 available before the stock started specialising along other lines ; there 

 may be differences in the genetic make-up or the environment of the two 

 forms which make such an adaptation less advantageous to one than to 

 the other. For instance, rare species are not likely to show the same 

 adaptations as abundant ones. 



