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Show 342 GEOLOGICAL SUCCESSION. CHAP. X. only certain classes of organic beings have been largely preserved in a fossil state; that th.e number both ~f specimens and of species, preserved. In our ~useums, Is absolutely as nothing compared with the Incalculable number of generations which must have passed away even during a single · formation; that, owing to subsidence being necessary for the accumulation of fossili-. ferous deposits thick enough to resist future degradation, enormous intervals of time have elapsed between the successive formations; that there has probably been more extinction during the periods of subsidence, and Inore variation during the periods of elevation, and durino- the la.tter the record will have been least per- o . fectly kept; that each single formation has not been continuously deposited; that the duration of each formation is, perhaps, short compared with the average duration of specific forms; that migration has played an important part in the first appearance of new forms in any one area and formation; that widely ranging species are those which have varied most, and have oftenest given rise to new species; and that varieties have at first often been local. All these causes taken conjointly, must have tended to make the geological record extren1ely imperfect, and will to a large extent explain why we do not find interminable varieties, conneeting together all the extinct and existing forms of life by the finest graduated steps. He who rejects these views on the nature of the geologieal record, will rightly rejec.t my whole theory. For he may ask in vain where are the numberless transitional links which must formerly have connected the closely allied or representative species, found in the several stages of the same great formation. He may disbelieve in the enormous intervals of time which have elapsed between our consecutive formations; he CHAP. X. SUMMARY. 34:) may overlook how important a part migration must hav.e played, when the formations of any one great regwn alone, as that of Europe, are considered; he may urge the apparent, but often falsely apparent, sudden coming in of whole groups of speeies. He n1ay ask where are the remains of those infinitely numerous organisms which must have existed long before the first bed of the Silurian system was deposited: I can ~nswer this latter question only hypothetieally, by sayIng that as far as we can see, where our oceans now extend they have for an enormous period extended, and where our oscillating continents now stand they have stood ever since the Silurian epoch· but that lono- ' 0 before that period, the world may have presented a wholly different aspect; and that the older continents, formed of formations older than any known to us, may now all be in a metamorphosed condition or may lie buried under the oeean. ' P~ssing fro:r:n these clifficul ties, all the other great leading facts In palreon tology seem to me simply to follow on the theory of descent with modification through natural selection. "\\T e can thus understand ~ow it is that new species come in slowly and suecessively ; how species of different classes do not necessarily ehange together, or at the sa1ne rate, or in the sam~ de?ree; yet in the long run that all undergo modification to some extent. The extinction of old form.s is the almost inevitable consequence of the productw~ of new forms. vV e can understand why when a species has once disappeared it never reappears. Groups of species increase in numbers slowly, and end~re f~r u~equal periods of time ; for the process of modification IS necessarily slow, and depends on many complex contingencies. The dominant species of the larger dominant groups tend to leave many modified |