CLASSIFICATION 383 



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 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 characters have especial 

 value in classification. 



We can understand why a species or a group of 

 species may depart, in several of its most important 

 characteristics, 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 community 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 con- 

 ditions of existence — are generally the most constant, 

 we attach especial 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 characters are of such high 

 classificatory importance. Geographical distribution 

 may sometimes be brought usefully into play in classing 

 large and widely-distributed genera, because all the 

 species of the same genus, inhabiting any distinct and 



