234 THE ORIGIN OF SPECIES 



and females have flourished, and transmitted to their fertile off- 

 spring a tendency to produce sterile members with the same modi- 

 fications. 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 produced which we see in 

 many social instincts. I 



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, some- 

 times to an almost incredible degree, and are thus divided into two 

 or even three castes. The castes, moreover, do not commonly grad- 

 uate into each other, but are perfectly well defined; being as dis- 

 tinct 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 ex- 

 traordinarily 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 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 from the 

 analogy of ordinary variations, that the successive, slight, profit- 

 able modifications did not first arise in all the neuters in the same 

 nest, but in some few alone; and that by the survival of the com- 

 munities with females which produced most neuters having the 

 advantageous modification, all the neuters ultimately came to be 

 thus characterized. According to this view we ought occasionally 

 to find in the same nest neuter insects, presenting gradations of 

 structure; and this we do find, even not rarely, considering how 

 few neuter insects out of Europe have been carefully examined. 

 Mr. F. Smith has shown that the neuters of several British ants 

 differ surprisingly from each other in size and sometimes in color ; 

 and that the extreme forms can be linked together by individuals 

 taken out of the same nest: I have myself compared perfect grada- 



