CHAP. VIII.] OF NATURAL SELECTION. 217 



of Myrmica. For the workers of Myrmica have not even rudi- 

 ments of ocelli, though the male and female ants of this genus 

 have well-developed ocelli. 



I may give one other case : so confidently did I expect 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 workersi 

 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 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 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 selection, 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 simultaneously 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 Miiller, 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, 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 



