350 CLASSIFICATION. [CHAP. XI\. 



several characters, let them be ever so trifling, concur throughout 

 a large group of beings having different habits, \ve 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 important character- 

 istics, 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 com- 

 munity 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 

 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 distinction between real affinities and 

 analogical or adaptive resemblances. Lamarck first called atten- 

 tion to this subject, and he has been ably followed by Macleay 

 and others. The resemblance in the shape of the body and in the 

 fin -like anterior limbs between dugongs and whales, and between 

 these two orders of mammals and fishes, are analogical. 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. Mivart, between the mouse and a small mar- 

 supial animal (Antechinus) of Australia. These latter resem- 

 blances may be accounted for, as it seems to me, by adaptation 

 for similarly active movements through thickets and herbage, 

 together with concealment from enemies. 



Amongst insects there are innumerable similar instances ; thus 

 Linnaeus, misled by external appearances, actually classed an 

 homopterous insect as a moth. We see something of the same 

 kind even with our domestic varieties, as in the strikingly similar 

 shape of the body in the improved breeds of the Chinese and 

 common pig, which are descended from distinct species; and in 

 the similarly thickened stems of the common and specifically dis- 



