Chap. VII. NEUTEE INSECTS. 239 



cocystus, 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 conclude 

 from the analogy of ordinary variations, that each suc- 

 cessive, slight, profitable modification did not probably 

 at first appear in all the individual neuters in the same 

 nest, but in a few alone ; and that by the long-continued 

 selection of the fertile parents which produced most 

 neuters with the profitable modification, all the neuters 

 ultimately came to have the desired character. On this 

 view we ought occasionally to find neuter-insects of the 

 same species, in the same nest, presenting gradations of 

 structure ; and this we do find, even often, considering how 

 few neuter-insects out of Europe have been carefully 

 examined. Mr. F. Smith has shown how surprisingly the 

 neuters of several British ants differ from each other in 

 size and sometimes in colour ; and that the extreme 

 forms can sometimes be perfectly linked together by in- 

 dividuals taken out of the same nest : I have myself 

 compared perfect gradations of this kind. It often 

 happens that the larger or the smaller sized workers 

 are the most numerous ; or that both large and small 

 are numerous, with those of an intermediate size 

 scanty in numbers. Formica flava has larger and 



