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Show 189l SPECULA.TIONS ON TH'E [Ch. XI. number, three or four of which are sometimes appropriated to one genus, as to the Balama' for exam. ple. Even thou.g h we conce d e, t11ere£1• ore ~ that the geooo -raplncal range of man. ne spect·e s · re extensive in general than that of the terrestnal, IS mo . . (the temperature of the sea bemg more um~orm, and the land 1· mpe d'm g 1e s s t11e mi'grations of th. e o.c eamc than the ocean those of the terrestrial,) yet we thmk 1t most ~roba.ble that th~ aquatic species far exceed in number the mhabttants of the land. Without insisting on this point, we may safely assume, as we before stated, that, exclusive of microscopic beings, there are between one and two millions of species now inhabiting the terraqueous globe; so, that if only one of these were to become extinct annually, and one new one were to be every year called into being, more than a million of years would be required to bring about a complete revolution in organic life. . We are not hazarding at present any hypothesis as to the probable rate of change, but none will den~ that when we propose as a mere speculation the annu~l bt~th and .the annual death of one species on the globe, we 1magme no shght degree of instability in the animate creation. If we divide the surface of the earth into twenty regions of equal area, one of these might comprehend a space of land and water ab~ut equal in dimensions to Europe, and might contain a twentieth part ~f the million of species which we will suppose to exist. ln tlus region one species only would, according to the rate of mortality before assumed, perish in twenty years, or only five out of fifty thousand in the course of a century. But as a co~~ iderable proportion of the whole would belong to ~he aquatic classes, with which we have a very imperfect acquamtance, we may exclude them from our consideration, and thus .one ~nly mio-ht be lost in about forty years among the terrestrtal tnbes. 0 Now the mammiferous quadrupeds m• G reat B 't · are only ri am to other terrestn.a l speci.e s of orgam·c b e·m g s ' both plants and animals in the proportion of about one to two hundred and eighty;' and taking this as a rude approxi·m atiO· n t o a general Ch. XI.] APPEARANCE OF NEW SPECIES. 183 standard, it would require more than eight thousand years before it wou~d come to the turn of this conspicuous class to lose one of their number even in a region of the dimensions of Europe. It is easy, t~erefore,. to conceive, that in a small portion of such an area, m ~ountnes, for example, of the size of England and F~ance, perwds o~ much greater duration must elapse before It would be possible to authenticate the first appearance of one of the larger plants and animals, assuming the annual birth and death of one species to be the rate of vicissitude in the animate creation throughout the world. The observations of naturalists may, in the course of future centuries~ accumulate positive data, from which an insight into the la~s which go.vern this part of our terrestrial system may be denved ; but~ m the present deficiency of historical records~ we have traced up the subject to that point where geological monuments alone are capable of leading us on to the discovery of ulterior trut!1~. To these, therefore, we must now appeal, carefully exanumng the strata of recent formation wherein the remains of li·ving species, both animal and vegetable, are known to occur. We must study these strata in strict reference to their chronoJog~cal order as deduced from their superposition, and other relations. From these sources we may learn which of the species, now our contemporaries, have survived the greatest revolutions of the earth's surface; which of them have co-existed with the greatest number of animals and plants now ex~inct, and which have made their appearance only when the ammate world had nearly attained its present condition. From such data we may be enabled to infer whether species have been called into existence in succession or aU at one period; whethe~ si?gly, or whether by groups simultaneously; ~het~er th: ant1qmty of man be as high as that of any of the mferwr bemgs which now share the planet with him, or whether the human species is one of the most recent of the whole. |