380 RECAPITULATION. [CHAP. XV. 



sitions, namely, that all parts of the organisation and instincts 

 offer, at least, individual differences that there is a struggle for 

 existence leading to the preservation of profitable deviations of 

 structure or instinct and, lastly, that gradations 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 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 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 

 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 

 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 domesti- 

 cation would likewise induce sterility in their modified descendants 



