OF NATURAL SELECTION. 257 



occasionally to find gradations of important structures 

 between the different castes of neuters in the same species, 

 that I gladly availed myself of Mr. F. Smith's offer of 

 numerous specimens from the same nest of the driver ant 

 (Anomma) of West Africa. The reader will perhaps best 

 appreciate the amount of difference in these workers by 

 my giving, not the actual measurements, but a strictly 

 accurate illustration : the difference was the same as if we 

 were to see a set of workmen building a house, of whom 

 many were five feet four inches high, and many sixteen feet 

 high ; but we must in addition 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 dif- 

 ferent 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 Sir J. Lubbock 

 made drawings for me, with the camera lucida, of the jaws 

 which I dissected from the workers of the several sizes. 

 Mr. Bates, in his interesting " Naturalist on the Amazons," 

 has described analogous cases. 



With these facts before me, I believe that natural selec- 

 tion, by acting on the fertile ants or parents, could form a 

 species which should regularly produce neuters, all of large 

 size with one form of jaw, or all of small size with widely 

 different jaws ; or lastly, and this is the greatest difficulty, 

 one set of workers of one size and structure, and simulta- 

 neously another set of workers of a different size and 

 structure ; a graduated series having first been formed, as 

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

 having been produced in greater and greater numbers, 

 through the survival of the parents which generated them, 

 until none with an intermediate structure were produced. 



An analogous explanation has been given by Mr. Wallace, 

 of the equally complex case of certain Malayan butterflies 

 regularly appearing under two or even three distinct female 

 forms ; and by Fritz M tiller, of certain Brazilian crustaceans 

 likewise appearing under two widely distinct male forms. 

 But this subject need not here be discussed. 



I have now explained how, I believe, the wonderful fact 

 of two distinctly defined castes of sterile workers existing 



