218 CLASSIFICATION. [Chap. XIV. 



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, 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 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 classificatory importance. Geographical dis- 

 tribution 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 distinction between 

 real affinities and analogical or adaptive resemblances. 

 Lamarck first called attention to this subject, and he 

 has been ably followed by Maeleay and others. The 

 resemblance in the shape of the body and in the fin-like 

 anterior limbs between dugongs and whales, and be- 

 tween these two orders of mammals and fishes, are ana- 

 logical. So is the resemblance between a mouse and a 

 shrew-mouse (Sorex), which belong to different orders; 

 and the still closer resemblance, insisted on by Mr. Mi- 

 vart, between the mouse and a small marsupial animal 



