Characters as Adaptive and Specific. 175 



quitting considerations of mere authority, I appeal 

 to the facts of nature themselves ; and will now 

 proceed, as briefly as possible, to indicate the result 

 of such an appeal. 



For the following reasons, that birds and mam- 

 mals seem to furnish the best field for testing the 

 question by direct observation. First, these classes 

 present many genera which have been more care- 

 fully worked out than is usually the case with 

 genera of invertebrates, or even of cold-blooded 

 vertebrates. Secondly, they comprise many genera 

 each including a large number of species, whose 

 habits and conditions of life are better known than 

 is the case with species belonging to large genera 

 of other classes. Thirdly, as birds and mammals 

 represent the highest products of evolution in respect 

 of organization, a more severe test is imposed than 

 could be imposed elsewhere, when the question is 

 as to the utility of specific characters; for if these 

 highest products of organization fail to reveal, in a 

 large proportional number of cases, the utility of their 

 specific characters, much more is this likely to be the 

 case among organic beings which stand lower in the 

 scale of organization, and therefore, ex hypothesi> 

 are less elaborate products of natural selection. 

 Fourthly, and lastly, birds and mammals are the 

 classes which Mr. Wallace has expressly chosen to 

 constitute his ground of argument with regard to 

 the issue on which we are now engaged. 



It would take far too long to show, even in epi- 

 tome, the results of this inquiry. Therefore I will 

 only state the general upshot. Choosing genera of 

 birds and mammals which contain a large number 



