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Show 322 GEOLOGICAL SUCCESSION, CnAr.X. between our consecutive formations; and in these intervals there may have been much slow extermination. 1\{oreover, when by sudden immigration or by unusually rapid development, many species of a n~w 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 commonly be allied, for they will partake of some inferiority in common. Thus, as it seems to me, the manner in which single species and whole groups of species become extinct, accords well with the theory of natural selection. We need not marvel at extinction; if we must marvel, let it be at our presumption in imagining for a moment that we understand the many complex contingencies, on which the existence of each species depends. If we forget for an instant, that each species tends to increase inordinately, and that some check is always in action, yet seldom perceived by us, the whole economy of nature will be utterly obscured. Whenever we can precisely say why this species is more abundant in individuals than that ; why this species and not another can be naturalised in a given country ; then, and not till then, we may justly feel surprise why we cannot account for the extinction of this particular species or group of species. On the Forms of Life changing almost simultaneously throughout the World.-Scarcely any palooontological discovery is more striking than the fact, that the forms of life change almost simultaneously throughout the world. Thus our European Chalk formation can be recognised in many distant parts of the world, under the most different climates, where not a fragment of the mineral chalk itself can be found ; namely, in North r CHAP. X. THROUGHOUT THE WORLD. 323 ~merica, in equatorial South America, in Tierra del Fuego~ at the Cape of Good I-Iope, and in the peninsula of ~ndi~. For. at these distant points, the organic remains In certain beds present an unmistakeable degree of resemblanc: to those of the Chalk. It is not that the same speCies are met with; for in some cases not one species is. ~dentically the same, but they belong to the s~me famih~s,. genera, and sections of genera, and so~etimes are Similarly. characterised in such trifling points as mere superfimal sculpture. Moreover other for~s, which. are not found in the Chalk of Europe, but ~hi?h occur In the formations either above or below, are similarly absent at these distant points of the world. In the several successive palooozoic formations of Russia :V es~ern Europe and North America, a similar parallel~ Ism m the ~or~s of lif~ has been observed by several authors : so It Is, according to Lyell, with the several ~uropean and North American tertiary deposits. Even If the few fossil species which are common to the Old and N ev; W ~rids be kept ~holly out of view, the general paralleh~m In the successive forms of life, in the stages of the WI~el y separa~ed palooozoic and tertiary periods, would still be manifest, and the se:v~ral formations could be easily correlate.d. . These observations, however, relate to the marine inhabitants of distant parts of the world : we have not sufficient data to judge whether the productions of the land and of fresh water change at distant points in the same parallel manner. vV e may doubt whether they have thus changed : if the Megatherium, Mylodon, Macrauchenia, an~ Toxodon had been brought to Europe fro~ La Plata, Without any information in regard to their geological position, no one would have suspected that they had coexisted with still living sea-shells; but as these anomalous monsters coexisted with the Masto~ |