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Show 314 GEOLOGICAL SUCCESSION. CHAP. X. d well with roy theory. I These several facts ac~o~ lopment, causing all the believe in no fixed law 0 t e~ange abruptly, or simul-inhabitants of a country! do c The 1~rocess of modi-t equa egree. r taneously, or 0 an 1 · 1 w The variability of each fication must b? ex~redme yds 0 t · of that of all others. . . quite In epen en f b species IS . b'l't be taken advantage o Y Wh th such var1a 1 1 Y . . b e er . nd whether the vanations e ac-natural selection, at lesser amount thus causing 1 t d t a grea er or ' cumu a e 0 t or lesser amoun t of modification in .t he va.r y-a. grea er' es d epend s on many complex contingenCies, Ing spem ' . b ·r t being of a beneficial nature, on -on the vana. 1 I y . on the rate of breeding, the power of Intercrossmg, . dit' f the h · hys1cal con wns 0 on the slowly c angin? lip the nature of the other t. and more espema Y on . ?oun ~y, 'th which the varying species comes Into Inhablt~~ts WI e it is b no means surprising that competlt~on. Henc 'n they same identical fornl much one species should retai. h . that it should change longer than others; or, If c ~ng~ng, hical distribution; W the same fact In geograp . less. e see. 1 d h lis and coleopterous Insects for inst~nce, In. the an t ~ffer considerably from their ofMadeua ~aving come otinent of Europe, whereas the nearest allies on the con . d naltered We . ll db' ds have reroaine u · roar1ne she s an u 1 uicker rate of can per~aps unde.r~tan~ ~he ~~;:r:~iJY ~rganised prochange In terrestna ~n In . d lower productions, ductions compared With ~ar~ne an£ the hio·her beings by the more complex re atw~s o dit' b of life as to their orgam.c and I. n or. g · amc con Ions of ' the . f h pter When many explained In a orroer c a . modified and iro-inhabitants of a country have bechome . . le of com-d t d n t e pr1nc1p proved, we can un ers an ' o all-important rela-petition, and on that of the. roa~~ t an form which tions of organism to organism, : fi d ~nd improved, does not become in some degree roo 1 e CHAP. X. GEOLOGICAL SUCCESSION. 315 will be liable to be exterminated. Hence we can see why all the species in the same region do at last, if we look to wide enough intervals of time, become modified; for those which do not change will become extinct. In members of the same class the average amount of change, during long and equal periods of time, may, perhaps, be nearly the same·; but as the accumulation of long-enduring fossiliferous formations depends on great masses -of sediment having been deposited on areas whilst subsiding, our formations have been almost necessarily accumulated at wide and irregularly intermittent intervals ; consequently the amount of organic change exhibited by the fossils embedded in consecutive formations is not equal. Each formation, on this view, does not mark a new and complete act of creation, but only an occasional scene, taken almost at hazard, in a slowly changing drama. We can clearly understand why a species when once lost should never reappear, even if the very same conditions of life, organic and inorganic, should recur. For though the offspring of one species might be adapted (and no doubt this has occurred in innumerable instances) to fill the exact place of another species in the economy of nature, and thus supplant it; yet the two forms-the old and the nevv-would not be identically the same; for both would almost certainly inherit different characters from their distinct progenitors. For instance, it is just possible, if our fantail-pigeons were all destroyed, that fanciers, by striving during long ages for the same object, might make a new bree ar y distinguishable from our present fantail; but if the parent rock-pigeon were also destroyed, and in nature we have every reason to believe that the parent-form will generally be supplanted and P2 |