m 



NATURAL SELECTION. 



siderable between the species of the same genus, and great 

 between the species of distinct genera. How do these lesser 

 differences become augmented into the greater difference? 

 How do varieties, or as I have called them incipient species, 

 become converted into true and well-defined species ? How has 

 each new species been adapted to the surrounding physical con- 

 ditions, and to] the other forms of life on which it in any way- 

 depends? We see on every side of us innumerable adapta- 

 tions 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 produces a gall, 

 on which the larva feeds ; but there is another insect (Miso- 

 campus) which deposits its eggs within the body of the larva 

 within the gall, and is thus nourished by its living prey ; so 

 that here a hymenopterous insect depends 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,— 

 that is, the augmentation of the slight differences character- 

 istic 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. We shall 

 therein see that all organic beings, without exception, tend to 

 increase at so high a ratio, that no district, no station, not even 

 the whole surface of the land or the whole ocean, would hold 

 the progeny of a single pair after a certain number of genera- 

 tions. 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 m a slight degree, owing to changes in the surrounding 



* Leon Dufour in < Annales des Scienc. Nat.' (3rd series, Zoolog.), torn. v. p. 6. 



