448 ftECAHfULATIOtf. 



natural selection : and one of the most curious of these is 

 the existence in the same community of two or three 

 denned castes of workers or sterile female ants ; but I have 

 attempted to show how these difficulties can be mastered. 



With respect to the almost universal sterility of species 

 when first crossed, which forms so remarkable a contrast 

 with the almost universal fertility of varieties when crossed, 

 I must refer the reader to the recapitulation of the facts 

 given at the end of the ninth chapter, which seem to me 

 conclusively to show that this sterility is no more a special 

 endowment than is the incapacity of two distinct kinds of 

 trees to be grafted together; but that it is incidental on 

 differences confined to the reproductive systems of the inter- 

 crossed species. We see the truth of this conclusion in the 

 vast difference in the results of crossing the same two species 

 reciprocally — that is, when one species is first used as the 

 father and then as the mother. Analogy from the consider- 

 ation of dimorphic and trimorphic plants clearly leads to 

 the same conclusion, for when the forms are illegitimately 

 united, they yield few or no seed, and their offspring are 

 more or less sterile ; and these forms belong to the same 

 undoubted species, and differ from each other in no respect 

 except in their reproductive organs and functions. 



Although the fertility of varieties when intercrossed, and 

 of their mongrel offspring, has been asserted by so many 

 authors to be universal, this cannot be considered as quite 

 correct after the facts given on the high authority of Gart- 

 ner and Kolreuter. Most of the varieties which have been 

 experimented on have been produced under domestication ; 

 and as domestication (I do not mean mere confinement) 

 almost certainly tends to eliminate that sterility which, 

 judging from analogy, would have affected the parent-species 

 if intercrossed, we ought not to expect that domestication 

 would likewise induce sterility in their modified descendants 

 when crossed. This elimination of sterility apparently fol- 

 lows from the same cause which allows our domestic animals 

 to breed freely under diversified circumstances ; and this 

 again apparently follows from their having been gradually 

 accustomed to frequent changes in their conditions of life. 



A double and parallel series of facts seems to throw much 

 light on the sterility of species, when first crossed, and of 

 their hybrid offspring. On the one side, there is good reason 

 to believe that slight changes in the conditions cf life give 

 vigor and fertility to ail organic beings. We know also 



