218 CLASSIFICATION. [Chap. XIV. 



important characteristics, and jet 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 eyer 

 so unimportant, betrays 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 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 characters are of such high classi- 

 ficatory 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 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 



