xvii.] THE PllOBLEM OF THE ORIGIN OF SPECIES. 217 



higher Articulata are descended from worms ; and I regard 



as an instance of reversion to the characteristics of worms, 



the fact that non-sexual generation occurs among some of 



the lower Crustacea, and also among some genera of winged Crustacea, 



insects. The larvae of Aphis, and of some Diptera, produce f^gects 



other larvte by non-sexual generation ; and some butterflies 



produce fertile eggs without being fertilized by the male. 



Having in this chapter brought forward evidence in 

 favour of the theory of the production of species by a 

 process of development, I intend in the next to begin the 

 consideration of the laws of that process. 



NOTE. 



In the chapter on the Laws of Habit, I have stated my belief Is there a 

 that there is no limit to the possible extent of variation, ™? , -^ , 

 if only a sufficient number of generations is allowed. This is I maintaiu 

 contrary to the general view, which is that the possible extent ^ "^ t^^S^- 

 of variation is rigidly confined within the limits of species. It opposition 

 has been generally believed till now that variation can only form ?°,V®i' 

 varieties — as, for instance, the domestic varieties of the dog and 

 of the pigeon have been formed by variation from their original 

 wild stocks; but that no aniount of variation, even though 

 acting through geological time, could possibly derive two such 

 species as the dog and the cat by descent from a single ancestor. 

 This view has been maintained by a singularly able writer on Argument 

 Darwin's theory in the North British Eeview for June 1867, ^^ ^^orih 

 M'ho makes the following appeal to the test of fact : — reviewer 



The dog varies much in size, as in all other characters, and ^^^ *^^ 

 there are some very small breeds of dogs. But the jDossible tive. 

 smallness of the dog appears to have reached its limit ; for very Limit of 

 small dogs are prized by fanciers, who are willing to jsay highly iu doo-s 

 for them, so that there is an inducement to breed smaller dogs ^^^^ ^^^^^ 

 than the smallest yet bred. The method of doing so would be, 

 of course, to select the smallest dogs to breed from, and then 

 select the smallest of the offspring. But this is not done, and 

 apparently cannot be done. The small breeds of dogs, though 

 they were originally jiroduced by variation from a much larger 



