Chap. XV. Recapitulation. 405 



is the existence in the same community of two or three defined 

 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 intercrossed 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 consideration of 

 dimorphic and trimorphic plants clearly leads to the same con- 

 clusion, 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 Gartner and Kolreuter. Most of 

 the varieties which have been experimented on have been pro- 

 duced 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 follows 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 of life give vigour and fertility to all 

 organic beings. We know also that a cross between the distinct 

 individuals of the same variety, and between distinct varieties, 

 increases the number of their offspring, and certainly gives to them 



