INSTINCT 235 



tions of this kind. It sometimes happens that the larger or the 

 smaller sized workers are the most numerous; or that both large 

 and small are numerous, while those of an intermediate size are 

 scanty in numbers. Formica flava has larger and smaller workers, 

 with some few of intermediate size; and in this species, as Mr. F. 

 Smith has observed, the larger workers have simple eyes (ocelli), 

 which, though small, can be plainly distinguished, whereas the 

 smaller workers have their ocelli rudimentary. Having carefully 

 dissected several specimens of these workers, I can affirm that the 

 eyes are far more rudimentary in the smaller workers than can be 

 accounted for merely by their proportionately lesser size; and I 

 fully believe, though I dare not assert so positively, that the work- 

 ers of intermediate size have their ocelli in an exactly interme- 

 diate condition. So that here we have two bodies of sterile workers 

 in the same nest, differing not only in size, but in their organs of 

 vision, yet connected by some few members in an intermediate 

 condition. I may digress by adding, that if the smaller workers 

 had been the most useful to the community, and those males and 

 females had been continually selected, which produced more and 

 more of the smaller workers, until all the workers were in this 

 condition, we should then have had a species of ant with neuters in 

 nearly the same condition as those of Myrmica. For the workers 

 of Myrmica have not even rudiments 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 occa- 

 sionally 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 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 



