164 Darwin, and after Darwin. 



have been specific characters ; and therefore there is 

 no real distinction to draw between natural selection 

 as a theory of species and as a theory of adaptations. 

 Well, if this objection were to be advanced, the 

 answer would be obvious. Although it is true that 

 every adaptive character which is now common to 

 a group of species must originally have been dis- 

 tinctive of a single parent species, it by no means 

 follows that in its first beginning as a specific character 

 it appeared in the fully developed form which it now 

 presents as a generic, family, ordinal, or yet higher 

 character. On the contrary, it is perfectly certain 

 that in the great majority of instances such cannot 

 possibly have been the case ; and the larger the group 

 of species over which any particular adaptive character 

 now extends, the more evidently do we perceive that 

 this character must itself have been the product of 

 a gradual evolution by natural selection through an 

 innumerable succession of species in branching lines. 

 The wing of a bird, for example, is an adaptive 

 structure which cannot possibly have ever appeared 

 suddenly as a merely specific character : it must have 

 been slowly elaborated through an incalculable number 

 of successive species, as these branched into genera, 

 families, and orders of the existing class. So it is 

 with other class distinctions of an adaptive kind; 

 and so, in progressively lessening degrees, is it with 

 adaptive characters of an ordinal, a family, or a generic 

 value. That is to say, in all cases where an adaptive 

 structure is common to any considerable group of 

 species, we meet with clear evidence that the structure 

 has been the product of evolution through the ancestry 

 of those species; and this evidence becomes in- 



