162 Darwin, and after Darwin. 



the foregoing definition of the theory in question 

 follows by way of necessity. The propositions are, 

 first, that natural selection is taken to be the 

 agency which is mainly, if not exclusively, con- 

 cerned in the evolution of adaptive characters : 

 secondly, that these characters, when evolved, are in 

 some cases peculiar to single species only, while in 

 other cases, and in process of time, they become 

 the common property of many species : thirdly, that 

 in cases where they are peculiar to single species 

 only, they constitute at all events one of the reasons 

 (or even, as the ultra-Darwinians believe, the only 

 reason) why the particular species presenting them 

 have come to be species at all. Now, these being 

 the propositions on which we are all agreed, it 

 obviously follows, of logical necessity, that the theory 

 in question is primarily one which explains the exis- 

 tence of adaptive characters wherever these occur ; 

 and, therefore, whether they happen to be restricted 

 to single species, or are common to a whole 

 group of species. Of course in cases where they 

 are restricted to single species, the theory which 

 explains the origin of these particular adaptations 

 becomes also a theory which explains the origin 

 of these particular species ; seeing that, as we are 

 all agreed, it is in virtue of such particular adapta- 

 tions that such particular species exist. Yet even 

 in these cases the theory is, primarily, a theory 

 of the adaptations in virtue of which the particular 

 species exists ; for, ex hypothesi, it is the adaptations 

 which condition the species, not the species the 

 adaptations. But, as just observed, adaptations may 

 be the con^on property of whole groups of species : 



