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Show GEOLOGICAL SUCCESSION. CHAP. X. exterminated by its improved offspring, it . is. quite incredible that a fantail, identical with t~e existi~g breed, could be raised from any other spemes of pigeon, or even from the other well-established races. of the do-roes tI.C p I'geon , for the. newly-f.o rmed fantail .w o.u ld be almost sure to inherit from Its new progenitor some slight characteris~ic differe~ces. . . Groups of spemes, that. Is, ge~era and famihes, f~llow the same general rules In thmr appe~rance and disappearance as do single spedes, changing more or less quickly, and in a greater or lesser de~ree. A group does not reappear after it has once disappeared; or its existence, as long as it lasts, is continuous. I ai~ aware that there are some apparent exceptions to this rule, but the exceptions are surprisingly few, so few that E. Forbes, Pictet, and Woodward (though all str;ngly opposed to such views as I maintain) admit its truth; and the rule strictly accords with my theory. For as all the species of the same group have descended from some one species, it is clear that as long as any species of the group have appeared in the long s~ccession of ages, so long must its members hav~ continuously existed, in order to have generated either new a~d modified or the saine old and unmodified forms. SpeCies of the genus Lingula, for instance, must have con~inuously existed by an unbroken succession of generatwns, from the lowest Silurian stratum to the present day. We have seen in the last chapter that the speci~s of a group sometimes falsely appear to have come In abruptly; and I have attempted to give an explanation of this fact, which if true would have been fatal to my views. But such cases are certainly exce~tional · the general rule being a gradual increase Ill numbe' r, till the group reaches I.t s maxi.m um, an d then, sooner or later, it gradually decreases. If the _CHAP. X, EXTINCTION. 317 number of the species of a genus, or the number of t~e genera. of a .family, be represented by a vertical hn~ of varyin? thi?knes~, crossing the successive geological formations In whwh the species are found the line will sometimes falsely appear to begin at its lower end, no~ in a sharp point, but abruptly ; it then gradually thwken.s upwards, sometimes keeping for a space of equal thwkness, and ultimately thins out in the upper beds, marking the decrease and final extinction of the species. This gradual increase in number of the species of a group .is strictly conformable with my theory ; as the species of the same genus, and the genera o.f the same family, can increase only slowly and progressively ; for the process of modification and the production of a number of allied forms must be slow and gradual, -one species giving rise first to two or three varieties, these being slowly converted into species which in their turn ·produce by equally slow steps othe; species, and so on, like the branching of a great tree from a single stem, till the group becomes large. On Extinetion.-W e have as yet spoken only incidentall! of the disappearance of species and of groups of species. On the theory of natural selection the extinction of old forms and the production of new and improved forms are intimately connected together. The old notion of all the inhabitants of the earth having ?een swept away at successive periods by catastrophes, IS very generally given up, even by those geoloo-ists El. b ' as Ie de Beaumont, Murchison, Barrande, &c., whose ~eneral views would naturally lead them to this concluSIOn. On the contrary, we have every reason to believe, from the study of the tertiary formations, that species and groups of species gradually· disappear, one after another, first from one spot, then from another, and |
