268 RECAPITULATION. [Chap. XV. 



in the state of perfection of each organ may have existed, 

 each good of its kind. The truth of these propositions 

 cannot, I think, be disputed. 



It is, no doubt, extremely difficult even to conjecture 

 by what gradations many structures have been perfected, 

 more especially amongst broken and failing groups of 

 organic beings, which have suffered much extinction; 

 but we see so many strange gradations in nature, that 

 we ought to be extremely 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 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 recapitula- 

 tion 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 repro- 

 ductive 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 conclusion, for when the forms are illegitimately 



