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Show GEOLOGICAL SUCCESSION. [CHAP. X. 280 of death-to feel no surprise at sicknehss't b:t ;,h~n ~he sick man dies to wonder and to suspect t a e Ie y some unknown' deed of violence. . d d h The theory of n~tural selection IS. groun e on t e b r f th t each new variety, and ultunate~y each new e 1 speC~i es I.S a pro duced and mainta. ined by· having so1tn"et' ad- ' those with which It comes 1nto cotnpe 1 1on; vanta~e over . . 1 .!': • d £ . an d t e con Sequent extinction. of ess-1avou't1he o1dm s almost inevitably follows. It IS the san;e w1 . our o-mestic productions :. wh~n a new and sbghtly Impro:ed variety has been raised, 1t at ~rst supplants the less 1m-roved varieties in the same neighbourhoo~; when much fmproved it is transported far and near, hke ou: shorthorned cattle, and takes the place of other breeds In other countries. Thus the appearance of new forms a~~ ~he disappearance of old forms, b~th natu;al. and art1fimal, are bound together. In certain flourishing groups, the nu1nber of new specific forms which have been produced within a given time is probably gr~ater than that o: the old forms which have been exterminated ; hr~.t we l~_now that the number of species has not gone. on 1nde~n1tely increasing, at least du~ing the later geol.ogiCal penods, so that looking to later times, we may beheye t?at the production of new forms has caused the extlnctlon of about the same number of old forms. The competition will generally be most severe, as formerly explained and illustrated by exa~ples, between the forms which are most like each other 1n all respec:s. Hence the improved and modified. des?endants of a spemes will generally cause the extermination of the parentspecies ; and if many new forms have been dev:lop~d from any one species, the nearest a~lies of that speCI.es, 11. e. th~ species ~f the same genus, ~111 be the most hable to ~xterminatlon. Thus, as I beheve, a number of new specieS descended from one species, that is a new genus, come~ to supplant an old genus, belonging to the san;e family. But it must often have happened th~t a new species belong~ ing to some on.e group ~Ill have s~Iz~d on the place occu pied by a species belonging to a distinct gro~p, and thus caused its extermination ; and if many alhed forms be CHAP. X.] EXTINCTION. 281 d~velope~ from the succe~sfu~ intruder, many will have to ywld thmr places ; and 1t Will generally be allied forms which will suffer from some inherited inferiority in com~ mon. But whether it be species belonging to the same or to a distinct class, which yield their places to other species which have been modified and improved, a· few of the su.ff~rers may often long be preserved, from beinO' fitted to some peculiar line of life, or from inhabiting so~e distant an~ ~solated sta~ion, where t~ey have e~caped severe competition. For Insta~ce, a smgle species 0~ Trigonia, a great genus of shells In the secondary formations survives in the .Australian seas ; and a few members of the great and almost extinct group of Ganoid fishes still inhabit our ~resh waters. Therefore the utter extinction of a group IS generally, as we have seen, a slower process than its production. With respect to the apparently sudden extermination of whole families or orders, as of Trilobites at the close of the palreozoic period and of Ammonites at the close of the secondary period, we must remember what has been already said on the probable wide intervals · of time between our consecutive formations ; and in these intervals there may have been much slow extermination. Moreover, when by sudden immigration or by unusually rapid develol?ment, many species of a new group have taken possession of a new area, they will have exterminated in a correspondingly rapid manner many of the old inhabitants; and the forms which thus yield their places will ~ommonly be allied, for they will partake of some inferiority In common. Thus, as it seems to 1ne, the manner in whjch sinO'le species and whole groups of species become extin~t accords well with the theory of natural selection. W ~ need not marvel at extinction; if we must marvel let it be at our presumption in imagining for a moment that we unders~and the many complex contingencies, on which the. existence of each species depends. If we forget for an mstant, that each species tends to increase inordinately an_d that some check is always in action, yet seldom per: ce1ved by us, the whole economy of nature will be utterly |