426 CLASSIFICATION. Chap. XIII. 



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 its 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, occur together throughout 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 correlated or aggregated cha- 

 racters have especial value in classification. 



We can understand why a species or a group of spe- 

 cies may depart, in several of its most important cha- 

 racteristics, from its allies, and yet be safely classed with 

 them. This may be safely done, and is often done, as 

 long as a sufficient number of characters, let them be 

 ever so unimportant, betrays the hidden bond of com- 

 munity of descent. Let two forms have not a single 

 character in common, yet if these extreme forms are 

 connected together by a chain of intermediate groups, 

 we may at once infer their community of descent, and 

 we put them all into the same class. As we find organs 

 of high physiological importance — those which serve to 

 preserve life under the most diverse conditions of exist- 

 ence — are generally the most constant, we attach espe- 

 cial value to them ; but if these same organs, in another 

 group or section of a group, are found to differ much, we 

 at once value them less in our classification. We shall 

 hereafter, I think, clearly see why embryological cha- 

 racters are of such high classificatory importance. 



