206 CLASSIFICATION. [Chap. XIV. 



would be of very high importance in classification. 

 Nothing can be more false. No one regards the ex- 

 ternal 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 con- 

 sideration 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 indica- 

 tions of its true affinities. We are least likely in the 

 modifications of these organs to mista]f;e a merely adap- 

 tive 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 significa- 

 tion; whereas the organs of reproduction, with their prod- 

 uct 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 pre- 

 served 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 



