CLASSIFICATION 227 



We can understand why a species or a grou[) 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 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 oi descent^ aud 

 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 distri- 

 bution may sometimes be brouglit 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 lie has been ably followed by 

 Macleay and others. The resemblance in the shape of 

 the body and in the finlike anterior limbs between du- 



