Chap. XIV.] CLASSIFICATION, 2l7 



which have undergone a certain, and sometimes a con- 

 siderable amount of modification, may not this same 

 element of descent have been unconsciously used in 

 grouping species under genera, and genera under higher 

 groups, all under the so-called natural system? I be- 

 lieve it has been unconsciously used; and thus only 

 can I understand the several rules and guides which 

 have been followed by our best systematists. As ve 

 have no written pedigrees, we are forced to trace com- 

 munity of descent by resemblances of any kind. There- 

 fore we chose those characters which are the least likely 

 to have been modified, in relation to the conditions 

 of life to which each species has been recently exposed. 

 Eudimentary structures on this view are as, good as, or 

 even sometimes better than, other parts of the organisa- 

 tion. We care not how trifling a character may be — let it 

 be the mere inflection of the angle of the jaw, the man- 

 ner in which an insect's wing is folded, whether the 

 skin be covered by hair or feathers — if it prevail 

 throughout many and different species, especially those 

 having very different habits of life, it assumes high 

 value; for we can account for its presence in so many 

 forms with such different habits, only by inheritance 

 from a common parent. We may err in this respect in 

 regard to single points of structure, but wlien several 

 characters, let them be ever so trifling, concur through- 

 out a large group of beings having different habits, we 

 may feel almost sure, on the theory of descent, that 

 these characters have been inherited from a common 

 ancestor; and we know that such aggregated characters 

 have especial value in classification. 



We can understand why a species or a group of 

 species may depart from its allies, in several of its most 



