ANALOGICAL RESEMBLANCES. 413 



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 suffi- 

 cient number of characters, let them be ever so unimpor- 

 tant, betray the hidden bond of community of descent, 

 i 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 importance — those 

 which serve to preserve life under the most diverse condi- 

 tions 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 distribu- 

 tion 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 proba- 

 bility descended from the same parents. 



ANALOGICAL RESEMBLANCES. 



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 resemblances 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 (Ant- 

 echinus) 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 



