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Show 426 CLASSIFICATION. CHAP. XIII. care not how trifling a character m.ay be-let it be t~e mere inflection of the angle of the .1aw, the manner Jn which an insect's wing is folded, whether the skin be covered by hair or feathers-if. it prevail thr?ughout many and different species, espeCially .those having very different habits of life, it assumes h1gh value ; for we can account for its presence in so many forms with such different habits, only by its inheritance from a common parent. We may err in this respect in regard to single points of structure, but when several characters, let them be ever so trifling, occur together throughout a large group of beings having different habits, we may feel almost sul'e, on the theory of descent, that these characters have been inherited from a common ancestor. And we know that such correlated or aggregated characters have especial value in classification. We can understand why a species or a group of spe-cies may depart, in several of its most important characteristics, from its allies, 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 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 her after, I think, clearly see why embryological cha~ racters are of such high classificatory importance. CHAP. XIII. CLASSIFICATION. 427 Geograp~ical distri?utiou ;may sometimes be brought usefully Into play In classing large and widely-distri? uted. ~enera, be~a~se all the species of the same genus, Inhabiting any distinct and isolated region, have in all probability descended from the same parents. We can understand, on these views, the very important distinction between real affinities and analoo·ical or adaptive resemblances. La~narck first called a~tention to this distinction, and he has been ably followed by Macleay and ?thers. The resemblance, in the shape of the body and In the fin-like anterior limbs, between the dugong, which is a pachydermatous animal, and the whale, and between both these mammals and fishes is analogical. An1ongst insects there are innumerable instances : thus Linnoous, misled by external appearances, actually classed an homopterous insect as a moth. We see something of the same kind even in our domestic varieties, as in the thickened stems of the common and swedish turnip. The resemblance of the greyhound and racehorse is hardly more fanciful than the analogies w~~h hav~ been drawn by some authors between very disti~ct animals. On my view of characters being of real Importance for classification, only in so far as they reveal descent, we can clearly understand why analoo·ical or adaptive character, although of the utmost import~nce to the welfare of the being, are almost valueless to the s!stematist. For animals, belonging to two most distinct hnes. ~f descent, may readily become adapted to similar conditions, and thus assume a close external resemblance; but such resemblances will not reveal-will rather tend to conceal their blood-relationship to their proper lines of descent. We can also under8tand the appare~t paradox, that the very same characters are analogical when one class or order is compared with another, but give true affinities when the rnembers of |