NATUEAL SELECTION. 5 



in some slight degree plastic; granting that many animals 

 and plants have varied greatly under domestication, and that 

 man by his power of selection has gone on accumulating such 

 variations until he has made strongly marked and firmly in- 

 herited races; granting all this, how, it may be asked, have 

 species arisen in a state of nature ? The differences between 

 natural varieties are slight ; whereas the differences are con- 

 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 1 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 the highest 

 admiration of every observer. 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, — has been briefly treated in my ' Origin of Species.' It 

 was there shown 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, 



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



