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Show 276 GEOLOGICAL SUOOESSION. [OHAP. X, new and complete act of creation, but only an occasio~al scene taken almost at hazard, in a slowly changing drama' . ' · h . We can clearly understand wh:y: a species w en once lost should never reappea~, even ;f the very same conditions of life, organic and Inorgan;c, sh~uld recur. For though the offspring of one SJ?ec;es might b~ adapted (and no doubt this has occurred In Inn~m~rable Instances) to fill the exact place of anot~er species In the economy of nature, and thus supplant It;, yet .the two forms-the old and the new-would not be Identically the same ; for both would almost certain~y inh~rit d.i:fferent c~a:ac~ers from their distinct progenitors. For Instance, 1t 1s JUSt possible if our fantail-pigeons were all destroyed, ~hat fanciers' by striving during long ages for the same obJect, might ~ake a new b~eed hardly distingui.shable from our present fantail; but 1f the parent rock-pigeon were ~lso destroyed and in nature we have every reason to beheve that the parent-form will generally be. s~ppl~nt~d and. exterminated by its improved o:ffsprmg, 1t IS quite Incredible that a fantail identical with the existing breed, could be raised from a~y other species of pigeon, or even from the other well-established races of the domestic pigeon, for newly-formed fantai.l would be. almost sure. t~ in~erit from its new progenitor some shght characteristic differ-ences. Groups of species, that is, genera and families, follow the same general rules in their appearance and disappearance as do single species, changing more or less quickly, and in a greater or lesser degree. A group does not reappear after it has once disappeared; .or its existence, as long as it lasts, i~ continuous. I am 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 strongly 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 succession of ages, so long must its members have contin- OIIAP. X.] EXTINOTION. 277 uous~y existed, in order to have generated either new and modified or the ~arne old ar:d unmodified forms. Species of the g~nus Lingula, for Instance, must have continuously existed by an unbroken succession of generations from the lowest Si~urian stratum to the present day. ' We have. seen .In the last chapter that the species of a grouf sometimes falsely ap_pear to have come in abruptly; an~ h.ave attempted to give an explanation of this fact, whiCh If true would have been fatal to my views. But ~uch cases are .certainl:y exceptional; the general rule beIng~ gradual Increase In number, till the group reaches its maxrmum, and then, sooner or later, it gradually decreases. If the number of the species of a genus or the nuffi:ber ?f the gen~ra of. a family, be represent~d by a vertiCa.l hne of v~ryin$ thic~ness, crossing the successive geolog.ICal form~tions In which the species are found, the line will . sometimes fa.lsely appear to begin at its lower en~, not 1n a sharp point? but abruptly; it then gradually th~ckens upwards, .sometimes .keeping for a space of equal thick~ess, and ultimately tlnns out in the upper beds ma.rking the ~ecrease ~nd :final extinction of the species~ ~his ~radualincrease In ~umber of the species of a group lS striCtly conformable With my theory; as the species of ~he same genus, and the genera of the same family, can 1ncr~ase ~nly 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 siowly converted Into species, which in their turn produce by equallJ: slow steps other species, and so on, like the branching of a great tree from a single stem, till the group becomes large. On Extinction.-W e have as yet spoken only inci ... dentall~ of the disappearance of species and of groups o.f speCies. On the theory of natural selection the extinc~ lOn of old. fo:;ms and the production of new· and improved frms are In~Imat~ly connected together. The old notion ~ all the Inh~bitants. of the earth having been swept way at succesrnve periods by catastrophes, is very gene- |
