XVII.] THE PROBLEM OF THE ORIGIN OF SPECIES. 209 



The same is true of the various breeds of the dog : no one, 

 merely by seeing them, would think that a greyhound and 

 a terrier were of the same species. It is quite possible — 

 though 1 only offer this as a suggestion for which it may Sugges- 

 never be possible to find proof — that mutual sterility ^jj),"g°^. 

 depends on the length of time that has elapsed since thejeot. 

 two stocks separated. The various races of pigeons have 

 all been produced under domestication, and consequently 

 during the historical period ; but the origin of wild races 

 may go back into geological time : so that, although the 

 most unlike races of pigeons are more visibly unlike than 

 the horse and the ass, the stocks of the horse and the ass 

 may have diverged from each other hundreds of thousands 

 of years before the stocks of the various races of pigeons 

 began to diverge ; and tliis may be the sole reason why 

 all breeds of pigeons are mutually fertile, whUe the horse 

 and the ass are not perfectly so. 



Another very obvious objection to the development Transi- 

 theory is, that if species have been formed by slow transi- fo^ms 

 tion, by descent, from one form to another, we ought to 

 find innumerable transitional forms ; but instead of finding 

 these, we find that each species is quite distinct. 



This is partly answered by the statement that in very 

 many cases each species is not quite distinct, and that we often still 

 do find a great variety of intermediate forms ; so that — what encT^'' ' 

 appears a strange paradox and yet is true — it often happens 

 that the more thoroughly a genus is known, the more 

 diflBcult it is to determine which of its members are species 

 and which only varieties.^ But this answer is utterly 

 insufiicient. If species have been formed by slow transi- but mostly 

 tion, only a very small proportion of all the forms that "^ ' 

 must have existed are now living. 



But why do we not find their remains entombed among 

 the rocks ? 



To this objection Darwin has, I think, given a conclusive 



it is well kuowu to be healthy, but not fertile. With cultivated plants 

 experiments of this kind are in general easy. 

 ^ Darwin's Origin of Species, p. 58, 



P 



