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Show 312 GEOLOGICAL SUCCESSION. CHAI'. X. CHAPTER X. ON THE GEOLOGICAL SuccESSION oF ORGANIC BEINGS. On the slow and successive appearance of new species- On their different rates of change- Species once lost do not reappear - Groups of species follow the same general rules in their appearance and disappearance as do single species- On ExtinctionOn simultaneous changes in the forms of life throughout the world- On the affinities of extinct S]_.)ccies to each other and to living species- On the state of development of ancient forms - On the succession of the same types within the same areasSummary of preceding and present chapters. LET us now see whether the several facts and rules relating to the geological succession of organic beings, better accord with the common view of the immutability of species, or with. that of their slow and gradual modification, through descent and natural selection. New species have appeared very slowly, one after another, both on the land and in the waters. Lyell has shown that it is hardly possible to resist the evidence on this head in the case of the several tertiary stages ; and every year tends to fill up the blanks between them, and to make the percentage system of lost and new forms more gradua:L In some of the most recent beds, though undoubtedly of high antiquity if measured by years, only one or two species are lost forms, and only one or two are new forms, having here appeared for the first time, either locally, or, as far as we know, on the face of the earth. If we may trust the observations of Philippi in Sicily, the successive changes in the marine inhabitants of that island have been many and most gradual. The secondary formations are more broken ; but, as Bronn has remarked, neither the appearance CHAP. X. GEOLOGICAL SUCCESSION. 313 nor di~appearance ~f their many now extinct species has been Simultaneous m each separate formation. Species of different genera and classes have not changed at th~ same rate, or in the same degree. In the old~st tertia~ beds a few living shells may still be found In the midst of a multitude of extinct forms. ~alconer .ha.s given a striking instance of a similar fact, In an existing crocodile associated with many strano-e and l?st ~amm~~s ~nd r~ptiles in the sub-Himalay~n deposits. The Silunan Lingula differs but little from the living species of this genus; · whereas most of the other Silurian Molluscs and all the Crustaceans have changed greatly. The productions of the land seem to change at a quicker rate than those of the sea of which a striking instance has lately been observed i~ Switzerland. There is some reason to believe that oro·anisms co~sidered high in the scale of nature, chan~e mor~ qmc~dy than ~hose that are low: though there are ex .. cepti?ns to this rule. The amount of organic change, as. Pwtet has r~marked, does not strictly correspond With the successiOn of our geological formations ; so that ~etween each two consecutive formations, the forms of life have seldom changed in exactly the saine degree. Yet .if we eompare any but the most closely related formatiOns, all the species will be found to have undergone some change . . When a species has once disappeared from the face of the earth, we have reas·on to believe that the same identical form never reappears. The strongest apparent exception to this latter rule, is that ?f the so-called "colonies" of M. Barrande, which mtrude for a period in the midst of an older formation. and t~en allow t?e pre-existing fauna to reappear; but Lyell s .expl~nation, namely, that it is a case of temporary migration from a distinct geographical province, seems to me satisfactory. p |