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Page:Popular Science Monthly Volume 34.djvu/249

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later Miocene and earlier Pliocene. There are the same species, the same mixture of laurels, hollies, olives, to which are added recent Japanese or Caucasian forms of nuts, maples, elms, and toward the high summits pines and firs very like those of the higher mountains of Teneriffe, Morocco, and Asia Minor. Collections made by M. Marion in the marly sediments of Durfort, where bones of the southern elephant were also found, and in the tufas of Valentine, near Marseilles, show that a number of plants now found only further south, still in the second half of the Pliocene inhabited the hills and shore-lines of southern France. These plants were then, therefore, at home further north than they are now found; and their occurrence in the Pliocene points to a later flowing back of species, amounting to a definite retreat in the Quaternary age.

This retreat, perfectly logical and almost regular in its operation, is connected with changes of climate, which were themselves in relation with a progressive depression of the temperature of the earth. It is connected also, in a parallel order of phenomena, with the exhaustion of some races, and with the development, by a concomitant origination, of other young and new races, favored by the same circumstances that caused the elimination of the races that gave way to them. Considering all the elements of the question, we find that it is by the extension, at a given moment, of vegetable races previously localized and realizing a certain amount of variation, that species are constituted at the start. Once characterized—that is, after the acquisition of a total of characteristics, at first fleeting, then hereditarily fixed—the species is permanent nevertheless only in a relative fashion so long as there exist in it parts susceptible of differentiation anew. The amplitude of the limits between which it may range through the course of time depends on the proportion of the elements that remain variable to those that will not change. The morphological oscillations of which it offers an example are thus determined by its own tendencies to submit more or less readily to excitations from without. Hence there are evident inequalities in the specific type, sometimes running to obscure shades, sometimes clearly cut; the last especially after the exclusion of intermediate forms.

There exist, in fact, fleeting species, which can not be circumscribed by any precise limit; and others, fixed in their minutest traits, that are susceptible only of insignificant variations. The forms of the latter category, such, for example, as the sequoias of America and the cedars of the Atlas, persist in the places of which they have once taken possession, where some of them have been driven back and cantoned, so that quite contrary conditions or the intrusion of more vigorous forms have not sufficed wholly