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Show 224 DARWINIANA. are just alike; and, even if some di:fferenc~ were discerned between them, it would not appreCiably alter the question as to how such a result came to pass. E h and every one of the analogous cases I have been d:t:iling-and very many more could be m~ntione~raises the same question, and would be satisfied wtth the same answer. These singular relations attracted my curiosity early in the course of my botanical studies, when c~mparatively few of them were known, and my senous attention in later years, when I had numerous and new Japanese plants to study in the coll~ctions made, by Messrs. Williams and Morrow, durmg Commodore Peny's visit in 1853, and especially, by Mr. Charles Wright, of Commodore Rodgers's expedition in 1.855. I then discussed this subject somewhat fully, and tabu-lated the facts within my reach.1 , This was before Heer had developed the rich fossil botany o:£ the arctic zone, before the im~ense antiquity of existing species o:£ plants was recogmzed, and before the publication o:£ Darwin's now :famous volum~. on the "Origin o:£ Species" had introduced and fam~harized the scientific world with those now current 1deas respecting the history and vicissitudes o:£ species with 'which I attempted to deal in a moderate and feeble way. . My speculation was based upon the :form~r glaCia-tion of the northern temperate zone, and the mference o:£ a warmer period preceding and perhaps ~ollowi~g. I considered that our own present vegetatwn, or 1ts proximate ancestry, must have occupied the arctic and 1 "Memoirs of American Academy," vol. vi., pp. 37'7-458 (1859). SEQUOIA AND ITS HISTORY. 225 subarctic regions in pliocene times, and that it had been gradually pushed southward as the temperature lowered and the glaciation advanced, even beyond its present habitation; that plants of the same stock and kindred, propably ranging round the arctic zone as the present arctic species do, made their forced migration southward upon wi,dely different longitudes, and receded more or less as the climate grew warmer; that the general difference o:£ climate which marks the eastern and the western sides of the continents-the one extreme, the other mean-was doubtless even then established, so that the same species and the same sorts of species would be likely to secure and retain foothold in the similar climates of Japan and the Atlantic United States, but not in · intermediate regions of different distribution of heat and moisture ; so that different species of the same genus, as in Torreya, or different genera o:£ the same group, as redwood, Taxodium, and Glyptostrobus, or different associations of forest-trees, might establish themselves each in the region best suited to the particular requirements, while they would fail to do so in any other. These views implied that the. sources 'Of our actual vegetation and the explanation of these peculiarities were to be sought in, and presupposed, an ancestry in plioc~ne or earlier times, occupying the higher northern regions. And it was thought that the occurrence of peculiar North American genera in Europe in. the Tertiary period (such as Taxodium, Carya, Liquidambar, sassa- . fras, N egundo, etc.) might be best explained on the assumption of early interchange and diffusion through North Asia, rather than by that of the fabled Atlantis. |