440 ANALOQIGAL RESEMBLANOEa. 



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 impor- 

 tance — 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 classificatory importance. 

 Geographical distribution may sometimes be brought use- 

 fully 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 RESEMBLAKTCBS. 



We can understand, on the above views, the very impor- 

 tant 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 (Antech- 

 inus) of Australia. These latter resemblances may be 

 accounted for, as it seems to me, by adaptation for similarly 

 active movements through thickets and herbage, together 

 with concealment from enemies. 



Among insects there are innumerable similar instances; 

 thus Linnaeus, misled by external appearances, actually 

 classed an homopterous insect as a moth. We see some- 

 thing of the same kind even with our domestic varieties, 

 as in the strikingly similar shape of the body in the im- 

 proved breeds of the Chinese and common pig, which are 

 descended from distinct species; and in the similarly thick- 



