214 ON THE ORIGIN OF SPECIES 



with social insects : a slight modification of structure, 

 or instinct, correlated with the sterile condition of 

 certain members of the community, has been advan- 

 tageous to the community : consequently the fertile 

 males and females of the same community flourished, 

 and transmitted to their fertile offspring a tendency to 

 produce sterile members having the same modification. 

 And I believe that this process has been repeated, 

 until that prodigious amount of difference between the 

 fertile and sterile females of the same species has been 

 produced, which we see in many social insects. 



But we have not as yet touched on the climax of the 

 difficulty ; namely, the fact that the neuters of several 

 ants differ, not only from the fertile females and 

 males, but from each other, sometimes to an almost 

 incredible degree, and are thus divided into two or 

 even three castes. The castes, moreover, do not 

 generally graduate into each other, but are perfectly 

 well defined ; being as distinct from each other, as are 

 any two species of the same genus, or rather as any 

 two genera of the same family. Thus in Eciton, there 

 are working and soldier neuters, with jaws and instincts 

 extraordinarily different : in Cryptocerus, the workers 

 of one caste alone carry a wonderful sort of shield on 

 their heads, the use of which is quite unknown : in 

 the Mexican Myrmecocystus, the workers of one caste 

 never leave the nest ; they are fed by the workers of 

 another caste, and they have an enormously developed 

 abdomen which secretes a sort of honey, supplying the 

 place of that excreted by the aphides, or the domestic 

 cattle as they may be called, which our European ants 

 guard or imprison. 



It will indeed be thought that I have an overweening 

 confidence in the principle of natural selection, when I 

 do not admit that such wonderful and well-established 

 facts at once annihilate my theory. In the simpler 

 case of neuter insects all of one caste or of the same 

 kind, which have been rendered by natural selection, 

 as I believe to be quite possible, different from the 

 fertile males and females, — in this case, we may safely 



