238 INSTINCT. Chap. VII. 



get nearly the same variety ; breeders of cattle 'wish the 

 flesh and fat to be well marbled together ; the animal 

 has been slaughtered, but the breeder goes with confi- 

 dence to the same family. I have such faith in the 

 powers of selection, that I do not doubt that a breed 

 of cattle, always yielding oxen with extraordinarily long 

 horns, could be slowly formed by carefully watching 

 which individual bulls and cows, when matched, produced 

 oxen with the longest horns ; and yet no one ox could ever 

 have propagated its kind. Thus I believe it has been 

 with social insects : a slight modification of structure, or 

 instinct, correlated with the sterile condition of certain 

 members of the community, has been advantageous to 

 the community : consequently the fertile males and 

 females of the same community flourished, and trans- 

 mitted to their fertile offspring a tendency to produce 

 sterile members having the same modification. And 

 I believe that this process has been repeated, until that 

 prodigious amount of difference between the fertile and 

 sterile females of the same species has been produced, 

 which we see in many social insects. 



But we have not as yet touched on the climax of the 

 difficulty; namely, the fact that the neuters of several 

 ants differ, not only from the fertile females and males, 

 but from each other, sometimes to an almost incredible 

 degree, and are thus divided into two or even three 

 castes. The castes, moreover, do not generally gra- 

 duate into each other, but are perfectly well defined ; 

 being as distinct from each other, as are any two species 

 of the same genus, or rather as any two genera of the 

 same family. Thus in Eciton, there are working and 

 soldier neuters, with jaws and instincts extraordinarily 

 different: in Cryptocerus, the workers of one caste alone 

 carry a wonderful sort of shield on their heads, the use 

 of winch is quite unknown: in the Mexican Blyrme- 



