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Show 296 GEOLOGICAL SUCCESSION. [CHAP. X. c1 e ary seen I. n the wonderful col.l ectiho n of fossfi l Bb one'1s made b MM. Lund and Clausen Ill t e caves o razi . I was sl much impressed with t~ese facts that I strongly l·l lSl·S t ed I· ll 1839 and 1845 ' on this " la• w ohf' th•e stu}c cession f t '"-on "this wonderful relations Ip 111 1e same o ypes, d h 1' . " p .(! continent between the dead an t c IVJng. r?1es~or Owen has subsequently extended the same generalisation to the mammals of the Old World .. We see tf1e sa!lle ~aw in this author's restoration ?f the e:ctinhct abJ?-ddgigfanthtlc buds of New Zealand. We see It also In t e u so e caves of Brazil. Mr. Woodward has shown th~t th~ sa~e l.aw holds good with sea-shells, bu~ fr?m the wide ~Istnbutlon of most genera of molluscs, It IS not well d1splaled by thmn. Other cases could be added, as the rela;tion between the extinct and living land-shells of Madeua; and between the extinct and living brackish-water shells of the Aralo-Caspian sea. . Now what does this remarkable law of the successiOn of the same types within the same are.as mean? l-Ie would be a bold man, who after companng th~ present climate of Australia and of parts of South Amenca under the same latitude, would attempt. t.o account, o~ tJ:w. on.e hand, by dissimilar physical conditions !or the dissimilarity of the inhabitants of these two continents, an~, on ~he other hand, by similarity of conditions, for th~ un1for:~rnty of the same types in each duri~1g.the l3:ter tertiary penods. Nor can it be pretended that It .Is an Immutable law th~t marsupials should have been chiefly or solely P!oduced In Australia · or that Edentata and other American types should h~ve been solely produced in South Amenca. For we know that Europe in ancient times ~as peopled. by numerous marsupials ; and ~ have s?own In the pu~hc~· tions above alluded to, that In Amenca the l~w of distri· bution of terrestrial mammals was formerly different from what it now is. North America formerly partook strongly of the present character of the southern half of the continent · and the southern half was formerly more closely allied,' than it is at present, to the northern half. , In. a similar manner we know from Falconer and Cautley s discoveries, that. northern India was formerly more closely CJIAP. X.] SAME TYPES IN SAME AREAS. 297 r~lated in its mammals to Africa t~an it .is at the present ti?le: ~nalogous ~acts c?uld be given In relation to the distnbution of me,nne anin1als. On the theory of descent with modification the great law of the long e;nd~1ring, but not immutable, s~ccession of ~he sam~ type.s within the san1e areas, is at once explained; f~r the Inhabitants of. each quarter of the world will obvwu~ ly ten~ to lea;e In that quarter, during the next succeeding P.e:Iod of time, closely allied though in some degree modified descendants. If the inhabitants of one cont~ nent forme~ly di~ered ~eatly from those of another continent, so Will thmr modified descendants still differ in ~early the sa~e manner and degree. But after very long 1nter~a~s of time. and a~ter great .geographical changes, permitting m~ch Inter-migration, the feebler will yield to the more. dominant forms, and there will be nothing immutable In the laws of past and present distribution. It may be asked in ridicule, whether I suppose that the ?Iegather~um and other a~lied huge monsters have left behmd them In South AmeriCa the sloth armadillo and anteater, as their degenerate descendant~. This c~nnot for an instant be admitted. These huge animals have become wholly exti~ct, and have left no progeny. But in the caves of B~·aZil_, th~re are many extinct species which are ?lose~y a!h~d 1.n size and in other characters to the spe~1es still hving In South America ; and some of these fossils may be the actual progenitors of living species. I~ must not be forgotten that, on Iny theory, all the speCies ?f the same genus have descended from some one spemes ; ~o that if six e;enera, each having eight species, be f~und In one geological forn1ation, and in the next succ. eeding format.ion there be six other allied or representa· tlve genera w1th the same number of species then we may conclude that only one species of each ~f the · six f~de~ genera has left modified descendants, constitutino-e SIX new genera. The other seven species of the ol~ gh~eha have all died out and have left no progeny Or fu Ic wo'?-ld J!robably be a far commoner case, t~o 0 ;. ·~iehpeCies of two or three alone of the six older genera WI ave been the parents of the six new genera· the 13* ' . f ~ I |