Seite - 462 - in The Origin of Species

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-