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Show I~"TTRODuCTION. NATURAL SELECTION. 5 siderable between the species of the same genus, and great between the species of distinct genera. How· do these lesser differen'ces become augmented into the greater difference ? How do varieties, or as I have called them incipient species, become converted iuto true and well-defined species?' How has each new species been adapted to the surrounding physical conditions, and to the other forms of life on which it in any way depcuds? We see on every side of us innumerable adaptations and contrivances, which have justly excited in the mind of every observer the highest admiration. There is, for instance, a fly (Cecidomyia) 3 which deposits its eggs within the stamens of a Scrophularia, and secretes a poison which produees a gall, on which the larva feeds; but there is another insect (Misocampus) which deposits its eggs within the body of the larva witltin the gall, and is thus nourished by its living prey ; so that here a hymenopterous insect depeuds on a dipterous insect, and this depends on its power of producing a monstrous growth in a particular organ of a particular plant. · So it is, in a more or less plainly marked manner, in thousands and tens of thousands of cases, with the lowest as well as with the highest productions of nature. ' This problem of the conversion of varieties into species,tbat is, the augmentation of the slight differences characteristic of varieties into the greater differences characteristic of species and genera, including the admirable adaptations of each being to its complex organic and inorganic conditions of life,will form the main subject of my second work. V\T e shall therein see that all organic beings, without exception, tend to increase at so high a ratio, that no distriet, no station, not even the whole surface of the land or the whole oceau, would hold the progeny of a single pair after a certain number of generations. The inevitable result is an ever-recurrent Struggle for Existence. It has truly been said that all nature is at war; the strongest ultimately prevail, the weakest fail; and we well know that myriads of forms have disappeared from the face of the earth. If then organic beings in a state of nature vary even in a slight degree, owing to changes in the surrounding 8 Leon Dufour in' Annalcs des Scienc. Nat.' (3rd series, Zoolog.), tom. v. p. 6. |