| OCR Text |
Show 318 GEOLOGICAL SUCCESSION. CHAP. X. finally from the world. Both single species. and whole groups of species last for very unequal penods ; some groups, as we have seen, having endured from the e.arliest lmown dawn of life to the present day; some having disappeared before the close of the palmozoic period. N 0 fixed law seems to determine the length of time durin()' which any single species or any single genus endui~s. 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, a~ of ammonites towards the close of the secondary penod, 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, T9xodon, and other extinct monsters which all co-existed with still living shells at a very l~te geological period, I w~s fil~ed. with astonishment; for seeing that the horse, since Its Introduction by the Spaniards into South Ameri<~a, has r~n wild over the whole country and has increased Ill numbers at an unparalleled rate, I asked myself what could so recently have exterminated the former horse under conditions of life apparently so favourable. But CHAP. X. EXTINCTION. 319 how utterly groundless was my astonishment 1 Prof~ ssor Owen soon perceived that the tooth, though so hke . that of the . existing horse, belonged to an extinct species. Had this horse been still living, but in some degree rare, no .naturalist would have felt the least surprise at its rarity ; for rarity is the attribute of a vast number of species of all classes, in all countries. ·If we ask ourselves why this or that species is rare we ans~er that something is unfavourable in its conditions of hfe ; but what tha~ ~omething is, we can hardly ever ~ell. On the suppositiOn of the fossil horse still existIng as a rare species, we might have felt certain from the analogy of all other mammals, even of the slow? re~ding elephant, and from the history of the naturalIsatwn of the domestic horse in South America, that under more favourable conditions it would in a very few years have stocked the whole continent. But we could not have told what the unfavourable conditions were which checked its increase, whether some one or several contingencies, and at what period of the horse's life, and in what degree, they severally acted. If ~he conditions had gone on, however slowly, becomIng less a~d less favourable, we assuredly should not h~ve perceived the fact, yet the fossil horse would certai~ y have become rarer and rarer, and finally extinct; -Its place being seized on by some more successful competitor. . It is most difficult always to remember that the Increase of every living being is constantly being checked by unperceived injurious agencies; and that these same unperceived agencies are amply sufficient to cause .rarity, and finally extinction. We see in many cases In the more recent tertiary formations, that rarity precedes extinction; and we know that this has been the progress of events with those animals which have |
