Chap. XIII. CLASSIFICATION. 415 



they may be for the welfare of the being in relation to 

 the outer world. Perhaps from this cause it has partly 

 arisen, that almost all naturalists lay the greatest stress 

 on resemblances in organs of high vital or physiological 

 importance. No doubt this view of the classificatory im- 

 portance of organs which are important is generally, but 

 by no means always, true. But their importance for 

 classification, I believe, depends on their greater con- 

 stancy throughout large groups of species ; and this con- 

 stancy depends on such organs having generally been 

 subjected to less change in the adaptation of the species 

 to their conditions of life. That the mere physiological 

 importance of an organ does not determine its classi- 

 ficatory value, is almost shown by the one fact, that in 

 allied groups, in which the same organ, as we have every 

 reason to suppose, has nearly the same physiological 

 value, its classificatory value is widely different. No 

 naturalist can have worked at any group without being 

 struck with this fact ; and it has been most fully ac- 

 knowledged in the writings of almost every author. It 

 will suffice to quote the highest authority, Robert 

 Brown, who in speaking of certain organs in the Pro- 

 teacese, says their generic importance, " like that of all 

 their parts, not only in this but, as I apprehend, in 

 every natural family, is very unequal, and in some cases 

 seems to be entirely lost." Again in another work he 

 says, the genera of the Connaraceee " differ in having 

 one or more ovaria, in the existence or absence of al- 

 bumen, in the imbricate or valvular aestivation. Any 

 one of these characters singly is frequently of more than 

 generic importance, though here even when all taken 

 together they appear insufficient to separate Cnestis from 

 Connarus." To give an example amongst insects, in 

 one great division of the Hymenoptera, the antennae, as 

 Westwood has remarked, are most constant in structure ; 



