MUTUAL AFFINITIES OF ORGANIC BEINGS 381 



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 

 conditions of life to which each species has been recently exposed. 

 Rudimentary structures on this view are as good, or even some- 

 times better than other parts of the organization. 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 ac- 

 count 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 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 aggregated char- 

 acters have especial value in classification. 



We can understand why a species or a group of species may de- 

 part from its allies, in several of its most important characteristics, 

 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, betray 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 inter- 

 mediate 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 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 

 presently see why embryological characters are of such high classi- 

 ficatory importance. Geographical distribution may sometimes be 

 brought usefully into play in classing large genera, because all the 

 species of the same genus, inhabiting any distinct and isolated 

 region, are in all probability descended from the same parents. 



ANALOGICAL RESEMBLANCES 



We can understand, on the above views, the very important dis- 

 tinction between real affinities and analogical or adaptive resem- 



