414 THE ORIGIN OF SPECIES 



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 consideration of di- 

 morphic and trimorphic plants clearly leads to the same conclu- 

 sion, 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 func- 

 tions. 



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 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 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 vigor 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 

 increased size and vigor. This is chiefly owing to the forms which 



