206 CLASSIFICATION. [Chap. XIV. 



nature, would be of very high importance in classifica- 

 tion. Nothing can be more false. No one regards the 

 external similarity of a mouse to a shrew, of a dugong 

 to a whale, of a whale to a fish, as of any importance. 

 These resemblances, though so intimately connected 

 with the whole life of the being, are ranked as merely 

 " adaptive or analogical characters ; " but to the 

 consideration of these resemblances we shall recur. It 

 may even be given as a general rule, that the less any 

 part of the organisation is concerned with special 

 habits, the more important it becomes for classification. 

 As an instance : Owen, in speaking of the dugong, 

 says, " The generative organs, being those which are 

 most remotely related to the habits and food of an 

 animal, I have always regarded as affording very clear 

 indications of its true affinities. We are least likely in 

 the modifications of these organs to mistake a merely 

 adaptive for an essential character." With plants how 

 remarkable it is that the organs of vegetation, on which 

 their nutrition and life depend, are of little signification ; 

 whereas the organs of reproduction, with their product 

 the seed and embryo, are of paramount importance ! 

 So again in formerly discussing certain morphological 

 characters which are not functionally important, we 

 have seen that they are often of the highest service in 

 classification. This depends on their constancy through- 

 out many allied groups ; and their constancy chiefly 

 depends on any slight deviations not having been 

 preserved and accumulated by natural selection, which 

 acts only on serviceable characters. 



That the mere physiological importance of an organ 

 does not determine its classificatory value, is almost 

 proved by the fact, that in allied groups, in which the 



