Chap. XIV. Analogical Resemblances. 373 



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 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 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 community 

 of descent. Let two forms have not a single character in com- 

 mon, 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 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 cha- 

 racters 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 

 attention 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 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. Mivart, between the mouse and 

 a small marsupial animal (Antechinus) of Australia, These latter 

 resemblances may be accounted for, as it seems to me, by adapta- 

 tion for similarly active movements through thickets and herbage, 

 together with concealment from enemies. 



Amongst insects there are innumerable similar instances ; thus 



