292 ORIGIN OF SPECIES 



with the longest horns; and yet no one ox would ever have 

 propagated its kind. Here is a better and real illustration : 

 according to M. Verlot, some varieties of the double annual 

 Stock from having been long and carefully selected to the 

 right degree, always produce a large proportion of seedlings 

 bearing double and quite sterile flowers ; but they likewise 

 yield some single and fertile plants. These latter, by which 

 alone the variety can be propagated, may be compared with 

 the fertile male and female ants, and the double sterile plants 

 with the neuters of the same community. As with the varie- 

 ties of the stock, so with social insects, selection has been 

 applied to the family, and not to the individual, for the sake 

 of gaining a serviceable end. Hence we may conclude that 

 slight modifications of structure or of instinct, correlated 

 with the sterile condition of certain members of the com- 

 munity, have proved advantageous : consequently the fertile 

 males and females have flourished, and transmitted to their 

 fertile offspring a tendency to produce sterile members with 

 the same modifications. This process must have been re- 

 peated many times, 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 acme of the diffi- 

 culty ; 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 commonly graduate 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 wonder- 

 ful sort of shield on their heads, the use of which is quite 

 unknown: in the Mexican Myrmecocystus, the workers of 

 one caste never leave the nest; they are fed by the workers 

 of another caste, and they have an enormously developed ab- 

 domen which secretes a sort of honey, supplying the place of 

 that excreted by the aphides, or the domestic cattle as they 

 may be called, which our European ants guard and imprison. 



