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Show 420 CLASSIFICATION. .CHAP. XIII. All the foregoing rules and aids and difficulties in classification are explained, if I do not great~y deceive myself, on the view that the natural system Is foun~ed on descent with modification; that the characters whiCh naturalists consider as showing true affinity between any two or more species, are those ",'hich have beon inherited from a common parent, and, In so far, all true classification is genealogical ; that community of descent is the hidden bond which naturalists have been unconsciously seeking, and not some unknown plan of creation, or the enunciation of general propositions, and the mere putting together and separating objects more or less alike. But I must explain my meaning more fully. I believe that the arrangement of the groups within each class, in due subordination and relation to the other groups, must be strictly genealogical in order to be natural; but that the amount of difference in the several branches or groups, though allied in the same degree in blood to their common progenitor, may differ greatly, being due to the different degrees of modification which they have undergone ; and this is expressed by the forms being ranked under different genera, families, sections, or orders. The reader will best understand what is meant, if he will take the trouble of referring to the diagram in the fourth chapter. vVe will suppose the letters A to L to represent allied genera, which lived during the Silurian epoch, and these have descended from a species which existed at an unknown anterior period. Species of three of these genera (A, F, and I) have transmitted modified descendants to the present day, represented by the fifteen genera ( a14 to zl4) on the uppermost horizontal line. Now all these modified descendants from a single species, are represented as related in blood or descent to the same CHAP. XIII. CLASSIFICATION. 421 degree; they may metaphorically be called cousins to the same millionth degree ; yet they differ widel and in different degrees from each other. The for:a descended from A, now broken up into two or three families, constitute a distinct order from those descended from I, also broken up into two families. Nor can the existing species, descended from A, be ranked in the same genus with the parent A; or those from I, with the parent I. But the existing genus F 14 may be supposed to have been but slightly modified· and it will the~ r~n.k with the parent-genus F; jdst as some few still hving organic beings belong to Silurian genera. So that the amount or value of the differences between organic beings all related to each other i~ the same degree in blood, has come to be widely different. Nevertheless their genealogical arrangem_ ent remains strictly true, not only at the present time, but at each successive period of descent. All the modified descendants from A will have inherited so.mething in common from their common parent, as Will all the descendants frorn I ; so will it be with each subordinate branch of descendants, at each successive period. If, however, we choose to suppose that any of the descendants of A or of I have been so much modified. as to have more or less completely lost traces · of their parentage, in this case, their places in a natural classificatio~ will have been more or less completely lost, -as ~omehmes seems to have occurred with existing organisms. All the descendants of the genus F alona • ' b Its whole line of descent, are supposed to have been but lit~le modified, and they yet form a single genus. ~ut th1s g~nus, tho~gh much isolated, will still occupy ~ts proper Intermediate position; for F originally was Intermediate in character between A and I, and the several genera descended from these two genera will |