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Show 344 GEOLOGICAL SUCCESSION. CHAP. X. descendants and thus new sub-groups and groups are formed. A~ these are formed, the species of the less vigorous groups, from their inferiority. inherited from a common progenitor, tend to become extinct together, and to leave no modified offspring on the face of the earth. But the utter extinction of a whole group of species may often be a very slow process, from the survival of a few descendants, lingering in protected and isola ted situations. When a group has once wholly disappeared, it does not reappear; for the link of generation has been broken. We can understand how the spreading of the dominant forms of life, which are those that oftenest vary, will in the long run tend to people the world with allied, but modified, descendants ; and these will generally succeed in taking the places of those groups of species which are their inferiors in the struggle for existence. Hence, after long intervals of time, the productions of the world will appear to have changed simultaneously. We c~n understand how it is that all the forms of life, ancient and recent, make together one grand system ; for all are connected by generation. We can understand, from the continued tendency to divergence of character, why the more ancient a form is, the more it generally differs from those now living. Why ancient and extinct forms often tend to fill up gaps between existing forms, sometimes blending two groups previously classed as distinct into one; but more commonly only bringing them a little closer together. The more ancient a form is, the more often, apparently, it displays characters in smne degree intermediate between groups now distinct; for the more ancient a form is, the more nearly it will be related to, and consequently resemble, the common progenitor of groups, since be- CHAP. x. SUMMARY. 345 come widely divergent. Extinct forms are seldom ~irectly i.ntermediate between existing forms; but are Intermediate only by a long and circuitous course through many extinct and very different forms. We can c!early see .why the organic remains of closely consecutive formations are more closely allied to each other, than are those of remote formations; for the forms are more closely linked together by generation: we can clearly see why the remains of an intermediate formation are intermediate in character. The inhabitants of each successive period in the world's h~story have beaten their predecessors in the race for hfe, and are, in so far, higher in the scale of nature ; an~ this may account for that vague yet illdefine. d s~ntiment, felt by many palooontologists, that organisatiOn on the whole has progressed. If it should hereafter be proved that ancient animals resemble to a certain extent the embryos of more recent animals of the .same class, the fact will be intelligible. The succession of the same types of structure within the same area~ during the later geological periods ceases to be mystenous, and is simply explained by inheritance. ~f the.n the geological record be as imperfect as I beheve It to be, and it may at least be asserted that the record cannot be proved to be much more perfect the main objections to the theory of natural selectio~ are greatly diminished or disappear. On the other ~a~d, an. the chief laws of palooontology plainly proc aim, as It se~ms to me, that species have been produced by ordinary generation: old forms having been supplanted by new and in1proved forms of life, produced by the laws of variation still acting round us, and preserved by Natural Selection. Q3 |