Chap. XIV.] CLASSIFICATION. 217 



which have undergone a certain, and sometimes a 

 considerable 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 

 beheve 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 we 

 have no written pedigrees, we are forced to trace 

 community of descent by resemblances of any kind. 

 Therefore we choose those characters which are the least 

 likely to have been modified, in relation to the con- 

 ditions 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 organisation. We care not how trifling a character 

 may be — let it be the mere inflection of the angle of the 

 jaw, the manner 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 when 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 



