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Show .- .... ~ ....... , · .. '·' . . / .. · ,, • , r'·~J·.· : ··' ·:·~· ~~ . ~ . . ~- 32-1: GEOLOGICAL SUCCESSION, CHAP. X. don and Horse,' it might at least have been. inferred that they had lived during one of the l~ter tertiary stages. When the marine forms of life are spoken of as having changed simultaneously .througho~t the world, it must not be supposed that this expression relates to the same thousandth or hundTed-thousandth year, or even that it has a very strict geological sense ; for if all the marine animals which live at the present day in Europe, and all those that lived in Europe duri~g the pleistocene period (an enormously remote perwd as measured by years, including the whole glacial epoch), were to be compared with those now living in South America or in Australia, the most skilful naturalist would hardly be able to say whether tlie existing or the pleistocene inhabitants of Europe resembled most closely those of the southern hemisphere. So, again, several highly competent observers believe that the existing productions of the United States are more closely related to those which lived in Europe during certain later tertiary stages, than to those which now live here ; and if this be so, it is evident that fossiliferous beds deposited at the present day on the shores of North America would hereafter be liable to be classed with somewhat older European beds. Nevertheless, looking to a remotely future epoch, there can, I think, be little doubt that all the more modern marine formations, namely, the upper pliocene, the pleistocene and strictly modern beds, of Europe, North and South America, and Australia, from containing fossil remains in some degree allied, and from not including those forms which are only found in the older underlying deposits, would be correctly ranked as simultaneous in a geological sense. The fact of the forms of life changing simultaneously, in the above large sense, at distant parts of the world, has greatly struck those adinirable observers, MM. CnAP. X. THROUGHOUT THE "'WORLD. 325 de V erneuil and d' Archiac. After referring to the parallelism of the palooozoic forms of life in various parts of Europe, they add, " If struck by this strange sequence, we turn our attention to North America, and there discover a series of analogous phenomena, it will appear certain that all these modifications of species, their extinction,_ and the introduction of new ones, cannot be owing to mere changes in marine currents or other causes more or less local and temporary, but depend on general laws which govern the whole animal kingdom." M. Barrande has made forcible remarks to precisely the same effect. It is, indeed, quite futile to look to changes of currents, climate, or other physical conditions, as the cause of these great mutations in the forms of life throughout the world, under the most different climates. We must, as Barrande has remarked, look to some special law. We shall see this more clearly w~en we treat of the present distribution of organic beings, and find how slight is the relation between the physical conditions of various countries, and the nature of their inhabitants. ~rhis great fact of the parallel succession of the forms of life throughout the. world, is explicable on the theory of ~at~ral s~l~ction. New species are formed by new vaneties ansing, which have some advantage over older forms; and those forn1s, whieh are already dominant, or have some advantage over the other forms in their own country, would naturally oftenest give rise to new varieties or incipient species; for these latter must be victorious in a still higher degree in order to be preserved and to survive. vVe have distinct evidence on thi~ head, in the plants which are don1inant, that is, wh1ch are commonest in their own homes and are most widely diffused, having produced the g1:eatest number of new varieties. It is also natural that the domi- |