462 ORIGIN OF SPECIES 



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, be- 

 trays 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 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 attention to this subject, and he has been ably fol- 

 lowed by Macleay and others. The resemblance in the shape 

 of the body and in the fin-like anterior limbs between du- 

 gongs and whales, and between these two orders of mam- 

 mals 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 marsupial 

 animal (Antechinus) of Australia. These latter resem- 

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



