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Show 316 CLASSIFICATION. [CHAP. XIII. t h m. r pn.m ora ·1 a1 parent , and every. intermedd iatt e link bin each branch and sub-branch of thmr .descen abn s, mfiay e supposed to be stl'1 1 a1 I' ve .' an d the hnksh 't o e a'st ne alds those between the :fi nest van·e t I' es. In . t. 1s bc ase h1' •who ntl be uite im ossible to give any definition y w Ic . _Ie sevqe ra1 memP b ers of the several .g roups cout ld be dtishh n-uished from their more immediate paren s;. o~ esc g f th ·r ancient and unknown progen1to1. Yet Parents rom ei • ld t'll h ld h t . 1 arr·anO'em ent in the diagram wou s l o t e na ur a o . · 11 th .!' goo d · an d , on the Principle of 1nhentance, a he ' 10rm· s d 'd d f m A or from I would have somet 1ng 111 escen e ro ' ' · · h t b h In a tree we can specify th1s or t a ranc , ctho mmho na. t the actual fork the two un1· te and ble nd t oge the 1·. w~u!ould not, as I have said, define the several. groups; but we could pick out types, or forms, representing most f the characters of each group, whether large ?r small, ~nd thus ~·ve a general idea of the value of the ~1fferenc~s b et ween t em . This is what we .s houlldl bhe .d!' nven• to, 1f we were ever to succeed in collecting a t. e 10rms 1n any class which have lived throughm;t all ~1me and s~ace. We shal1 certainly never succeed 1n maJnng so perfe~t a collection : nevertheless, in certain classes, we are .ter:-d1ng in this direction; and Milne .Ed~ards has lately 1n~1sted, in an able paper, on the h1gh Importance of lo~lnng to types whether or not we can separate and define the groups to which such types belong. . . Finally we have seen that natural selectl?n, whiCh results fro~ the struggle ~or existeD;ce, and wh1ch almos~ inevitably induces extinction and d1verg~nce of charactei in the many descendants from one dominant pare?t-species, explains that great and universal. feature !n ~he affinities of all organic beings, namely, the1r subord1natl~n in rou under group. We use the element of descent 1n cla~sin/ the individuals of both sexes and of all ages, although having few characters in common, under on~ species ; we use descent in classing acknow ~edged v~net~si however different they may be from their parent , an believe this element of descent is the hidden bond of conf nexion which naturalists have sought under the 1ter~ 0 the Natural System. On this idea of the natura sys ero OsAP. XIII.J MORPHOLOG-Y. 311 ?eing, in so far as .it has been perfected, genealogical in Its arrangement, w1th the grades of difference between the descend~nt~ from.~ common parent, expressed by the terms ge?-ma, famihes, orders, &c., we can understand the rules whiCh we are compelled to follow in our classification. We can understand why we value certain resemblances .far more than others ; why we are permitted to use rudimentary and useless organs, or others of triflinO' p~ysiolo~i~al importance; why, ill: comparing one group with a d1st1nct group, we summanly reject analogical or adaptive characters, and yet use these san1e characters within the limits of the same group. We can clearly see how it js that all living and extinct forms can be grouped together in one great system; and how the several members of each class are connected together by the most complex and radiating lines of affinities. We shall never probably, disentangle the inextricable web of affinitie~ between the members of any one class ; but when we have a distinct object in view, and do not look to some unknown plan of creation, we may hope to make sure but slow progress. lrforphology.-W e have seen that the members of the same class, independently of their habits of life, resemble each other in the general plan of their organisation. This resemblance is often expressed by the term "unity of type;" or by saying that the several parts and organs in the different species of the class are homologous. The whole subject is included under the general name of Morphology. This is the most interesting department of natural history, and may be said to be its very soul. What can be more curious than that the hand of ~ man, formed for grasping, that of a mole for digging, the leg of the horse, the paddle of the porpoise, and the wing of the bat, should all be constructed on the same pattern, and should include the sa~e bones, in the same relative positions ? Geoffroy St. Hilaire has insisted strongly on the high importance of relative connexion in homologotlS organs: the parts may change to almost any extent in form and size, and yet they always remain connected together in the same order. |