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Show 432 CLASSIFICATION. CHAP. XIII. together by a long, but broken, chain. of afiinities. Extinction has only separated groups: It has by no means made them; for if every form which has ever lived on this earth were suddenly to reappear, though it would be quite impossible to give definitions by which each group could be distinguished from other groups, as all would blend together by steps as fine as those be~ tween the finest existing varieties, nevertheless a natural classification, or at least a natural arrangement, would be possible. \¥ e shall see this by turning to the diagram : the letters, A to L, may represent eleven Silurian genera, some of which have produced large groups of modified descendants. Every intermediate link between these eleven genera and their primordial parent, and every intermediate link in eaeh branch and sub-branch of their descendants, may be supposed to be still alive; and the links to be as fine as those between the finest varieties. In this case it would be quite impossible to give any definition by which the several members of the several groups could be distinguished fron1 their more immediate parents; or these parents from their ancient and unknown progenitor. Yet the natural arrangement in the diagram would still hold good ; and, on the prin~ ciple of inheritance, all the forms descended from A, or from I, would have something in common. In a tree we can specify this or that branch, though at the actual fork the two unite and blend together. V\T e could not, as I have said, define the several groups; but we could pick out types, or forms, representing most of the cha~ racters of each group, whether large or small, and thus give a general idea of the value of the ~ifferenc~s between them. This is what we should be dnven to, If we were ever to succeed in collecting all the forms in any class which have lived throughout all time ~nd space. We shall certainly never succeed in makmg CHAP. XIII. CLASSIFICATION. 433 so perfect a collection : nevertheless in certain l t d. . . ' c asses, we are en. In. g In this direction ·' and Mil ne Ed d i war s has lately In~Isted, in an able paper, on the high import-ance of looking to types, whether or not we can separate and define the groups to which such types belong Finally, we have seen that natural selection,. which :esu~ts fro~ the struggle for existence, and which almost ~nevitably Induces extinction and divergence of character In t~e many. descendants from one dominant parentspec~ e~, explains that great and universal feature in the a!fini~Ies of all organic beings, namely, their subordinatiOn In group under group. We use the element of descent in classing the individuals of both sexes and of all ages, although having few characters in common under one. sp.ecies ; we use descent in classing acknow~ ledged vaneties, however different they may be from their parent; and I believe this element of descent is the hidden bond of connexion which naturalists have sought under the term of the Natural System. On this idea of the natur~l sr:st~m being, in so far as it has been perfected, genealogwal m Its arrangement, with the grades of difference between the descendants from a common parent, expressed by the terms genera, families, orders, &c., we can u~derstand t~e rules which we are compelled to follow In o~r classification. We can understand why we value certa1~ resemblances far more than others; why we are permitt~d.to use rudimentary and useless organs, or othe:s of trifbng physiological importance; why, in co~ paring o~e group with a distinct group, we summarily reJect analogical or adaptive characters, and yet use these same characters within the limits of the same group. ': e can clearly see how it is that all living and extinct orms can be grouped together in one great system · and how the several members of each class are con: nected together by the most complex and radiating u |