BEOAPITULATION. 47'? 



to_ be extremely cautious in saying that any organ or in- 

 stinct, or any whole structure, could not have arrived at 

 its present state by many graduated steps. There are, it 

 must be admitted, cases of special difficulty opposed to the 

 theory of natural selection: and one of the most curious of 

 these 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 in- 

 tercrossed 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 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. 



A-lthough 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 Kolreu fcer. 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-spe- 

 cies if intercrossed, we ought not to expect that domesti- 

 cation would likewise induce sterility in their modified 

 descendants when crossed. This elimination of sterility 

 apparently follows from the same cause which allows our 



