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Show 31:0 GEOLOGICAL SUCCESSION. CHAP. X. and, on the other hand, by similarit.y of conditi?ns, ~r h . £ . t f the same types In each dunng t e t e uni o:·mi y o. d Nor can it be pretended that it later tertiary perw s. b is an immutable law that marsupials .should have een chiefly or solely produced in Australia; or that Edentata and other American types should have been~ solely pro d uce d I·n South America · For we know that Eur· o1p e in ancient times was peopled by numerous marsupia s; d I have shown in the publications above alluded ~o, tahn t ·11 a America the law of distribution of terrestnal I h . . mammals was formerly different from w at It now Is. North America formerly partook strongly of th~ present character of the southern half of the contine~t; and the southern half was formerly more closely allied, than it is at present, to the northern half. In a ~imil~r manner we know from Falconer and Cautley s discoveries that northern India was formerly more closely related in its mamn1als to Africa than it is at the present time. Analogous facts could be given in relation to the distribution of marine animals. On the theory of descent with modification, the gr~at law of the long enduring, but not immutable, .successiOn of the same types within the same areas, IS at once explained ; for the inhabitants of eac~ quarter of ,the world will obviously tend to leave In that quarter, during the next succeeding per~od of time, closely allied though in some degree modified descendan~s. If the inhabitants of one continent formerlY . drffere.d greatly from those of a~oth~r con.tinent, so will then Inodified descendants still differ In nearly the same manner and degree. But after very long interv~ls. of time and after great geographical changes, permitting much inter-migration, the feebler will yield. to ~he more dominant forms, and there will b~ n?thi~g lffi .. mutable in the laws of past and present distnbutwn. CHAP. X. SAME TYPES IN SAME AREAS. 3-±1 It may be. asked in ridicule, whether I suppose that the me~athenum .and other allied huge monsters have left behind them In South America the sloth, armadillo, and anteater, as their degenerate descendants. This cannot for an instant be admitted. These huge animals have be~ome wholly extinct, and have left no progen. y. But In the caves of Brazil, there are manv extinct species which are closely allied in size and i~ other. characters to the species still living in South AmeriCa; and some of these fossils may be the actual progenitors of living species. It must not be forgotten that, on my theory, all the species of the same ?e~us have descended from some one species; so that If SIX genera, each having eight species, be found in one ~eo logical formation, and in the next succeeding formati? n there be six other allied or representative genera With the same nurnber of species, then we may conclude that only one species of each of the six older genera has left modified descendants constitutinO' tho . ' 0 SIX new genera. The other seven species of the old ge~era have all died out and have left no progeny. Or, whwh would probably be a far commoner case two or three species of two or three alone of the six older genera will have been the parents of the six new genera ; the other old species and the other whole genera having become utterly extinct. In failinoorders, with the genera and species decreasinO' i~ numbers, as apparently is the case of the Edentafa of South America, still fewer genera and species will have left modified blood-descendants. Summary of the preceding and present Oltapters.-I have atte~pted to show that the geological record is extremely Imperfect; that only a small portion of the globe has been geologically explored with care; that |