Chap. VII. NEUTER INSECTS. 241 



a house of whom many were five feet four inches high, 

 and many sixteen feet high ; but we must suppose that 

 the larger workmen had heads four instead of three 

 times as big as those of the smaller men, and jaws nearly 

 five times as big. The jaws, moreover, of the working 

 ants of the several sizes differed wonderfully in shape, 

 and in the form and number of the teeth. But the 

 important fact for us is, that though the workers can be 

 grouped into castes of different sizes, yet they graduate 

 insensibly into each other, as does the widely-different 

 structure of their jaws. I speak confidently on this 

 latter point, as Mr. Lubbock made drawings for me 

 with the camera lucida of the jaws which I had dis- 

 sected from the workers of the several sizes. 



With these facts before me, I believe that natural 

 selection, by acting on the fertile parents, could form 

 a species which should regularly produce neuters, 

 either all of large size with one form of jaw, or all of 

 small size with jaws having a widely different structure ; 

 or lastly, and this is our climax of difficulty, one set of 

 workers of one size and structure, and simultaneously 

 another set of workers of a different size and structure ; 

 — a graduated series having been first formed, as in 

 the case of the driver ant, and then the extreme forms, 

 from being the most useful to the community, having 

 been produced in greater and greater numbers through 

 the natural selection 'of the parents which generated 

 them ; until none with an intermediate structure were 

 produced. 



Thus, as I believe, the wonderful fact of two distinctly 

 defined castes of sterile workers existing in the same 

 nest, both widely different from each other and from 

 their parents, has originated. We can see how useful 

 their production may have been to a social community 

 of insects, on the same principle that the division of 



M 



