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Show 218 GEOLOGICAL SUCCESSION. [CHAP. X. rally given up, even by those geologists, as ·Elie de B~aumont, Murchison, Bar~·ande, &c., whose g.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 finally from the world. Both single species and whole groups of species last for very unequal periods; some groups as we have seen, having endured from the earliest known dawn of life to the present day; some having_ disappeared before the close of the palreozoic period. No fixed law seems to determine the length of tin1e during which any single species or any single genus endures. There is reason to believe that the complete extinction of the species of a group is generally a slower process than their production: if the appearance and disappearance of a group of species be represented, as before, by a vertical line of varying thickness, the line is found to taper more gradually at its upper end, which marks the progress of extermination, than at its lower end, which marks the first appearance and increase in numbers of the species. In some cases, however, the extermination of whole groups of beings, as of ammonites towards the close of the secondary period, has been wonderfully sudden. The whole subject of the extinction of species has been involved in the most gratuitous mystery. Some authors have even supposed that as the individual has a definite length of life, so have species a definite duration. No one I' think can have marvelled more at the extinction of species, than I have done. When I found in La Plata the tooth of a horse embedded with the remains of Mastodon, Megatherium, Toxodon, and other extinct monsters, which all co-existed with still living shells at a very late geological period, I was filled with astonishment ; for seeing that the horse, since its introduction by the Span· iards into South America, has run wild over the whole country and has increased in numbers at an unparalleled ra~e, I asked n1yself what could so ~e~ently have extermmated the former horse under conditions of life appar- CHAP. X.] EXTINCTION. 219 ently so favourable. But how u 1 · astonishment I Professor Owen~~~:~ groun~1ess was my tooth, thou. gh so like that of the exi.s t•I npger hcomrsv ed bt h1a t the to an extinct species. !-lad this h b e,. e ?n.ged but in some degree rare no natural_orse een still hving, least surprise at its rarity ; for rari:st .w~~ld ha~e felt the vast number of species of all cl y _IS lle attribute of a we ask ourselves why this or tha:sses, .In ~ countries. If that something is unfavourable ~~e?;es IS rda.r~, we ans:ver but what that somethin is . I s con Itlons of hfe; the ~uppositi~n of the f~ssii h~r~:~~llr~l!. ~yer tell. On species, we might have felt certain f ~his Ing as a rare other mammals, even of the slow-bOI~· e ayalogy of all from the history of the natur 1. r~e Ing e ephant, and horse in South America that ~ Isation of the domestic ditions it would in a ;er £ un er more. favourable conwhole continent But Y. eM years have stocked the unfavourable co~ditionsw;e~~~hi:t :aie Joid ~hat the whether some one or several . c ec . e Its Increase, period of the horse's life and .contng~Cies, and at what ally acted. If the condition hndw at egree, they severly, becoming less and less fa~ a ~one on, however slownot have perceived the factour~b t~ w£ as.suredly should certainly have become ra ' ye e ossil horse would tinct ;-its place being seiz:~ an~ rarer, and finally excoft~ titor. on Y some more successful tIS most difficult always t . b of every living being is consta o tl ebe~ erhthat the increase ceive~ injurious agencies. and ~l y t thng c ecked by u~peragencies are anlply su:ffi~ient t la ese sal?e unpercmved extinction. We see in man o caus~ rarity, and finally tertiary formations that raril cases In the . mo~e recent we know that this' has b y precedes extinction; and those animals which h ~en the progress of events with or ":holly, through ;::, een exterminated, either locally pubhshed in 1845 nam!l ag~hcy. I maJ:' repeat what I generally become ~a y, at to admit that species feel no surprise at :he ~~~~e t~ey becol?e extinct-to marvel greatl h . Y o a species, and yet to as to admit th!t :c:~~:: i~efues. t~.e~dist, 1i~ much the same · e In IVI ua IS the forerunner |
