500 ORIGIN OF SPECIES 



tremely cautious in saying that any organ or instinct, 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 selec- 

 tion ; 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 conclu- 

 sively to show that this sterility is no more a special endow- 

 ment 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 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 Gartner 

 and Kolreuter. Most of the varieties which have been ex- 

 perimented 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 inter- 

 crossed, 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 



