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Show 12 DARWINIAN A. and animal in a primitive stock, which reproduces its like from generation to generation, and so continues the species.1 Taking the idea of species from this perennial succession of essentially similar individuals, the chain is logically traceable back to a local origin in a single stock, a single pair, or a single individual, from which all the individuals composing the species have proceeded by natural generation. Although the similarity of progeny to parent is fundamental in the conception of species, yet the likeness is by no means absolute; all species vary more or less, and some vary remarkably-partly from the influence of altered circumstances, and partly (and more really) from unknown constitutional causes which altered conditions favor rather than originate. But these variations are supposed to be mere oscillations from a normal state, and in Nature to be limited if not transitory ; so that the primordial differences between species and species at their beginning have not been effaced, nor largely obscured, by blending through variation. Consequently, whenever two reputed species are found to blend inN ature through a series of intermediate forms, community of origin is ·inferred, and all the forms, however diverse, are held to belong to one species. Moreover, since bisexuality is the rule in Nature (which is practically carried out, in the long-run, far more generally than has been suspected), and the heritable qualities of two distinct individuals are mingled in the offspring, it is supposed that the genaral t "Species tot sunt, q~ot diversas formas ab initio produxit Iufinitum Ens ; qure formre, secundum generationis inditas leges, produxere plures, at sibi semper similes."-.Linn. Phil. Bot., 99, 157. THE ORIGIN OF SPECIES. 13 sterility of hybrid progeny interposes an effectual barrier against the blending of the original species by crossing. From this generally-accepted view the well-known theory of Agassiz and the recent one of Darwin diverge in exactly opposite directions. That of Agassiz differs fundamentally from the ordinary view only in this, that it discards the idea of a common descent as the real bond of union among the individuals of a species, and also the idea of a local origin-supposing, instead, that each species originated simultaneously, generally speaking, oyer the whole geographical area it now occupies or has occ:upied, and in perhaps as many individuals as it numbered at any subsequent period. Mr. Darwin, on the other hand, holds the orthodox view of the descent of all the individuals of a species not only from a local birthplace, but from a single ancestor or pair; and that each species has extended and established itself, through natural agencies, wherever it could; so that the actual geographical distribution of any species is by no means a primordial arrangement, but a natural result. He goes .farther, and this volume is a protracted argument intended to prove that the species we recognize have not been independently created, as such, but have descended, like varieties, from other species. Varieti~, on this view, are incipient or possible species : species are varieties of a larger growth and a wider and earlier divergence from the parent stock ; the difference is one of degree, not of kind. The ordinary view-rendering unto Cmsar the |
