Chap. VIII.] OF NATUEAL SELECTION. 359 



mitted to their fertile offspring a tendency to produce 

 sterile members with the same modifications. This 

 process must have been repeated many times, until 

 that prodigious amount of difference between the fertile 

 and sterile females of the same species has been pro- 

 duced, which we see in many social insects. 



But we have not as yet touched on the acme 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 commonly 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 Myrmeco- 

 cystus, 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 and 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 the theory. In the simpler case 

 of neuter insects all of one caste, which, as I believe, 

 have been rendered different from the fertile males and 

 females through natural selection, we may conclude 

 17 



