OF NATURAL SELECTION. 255 



to the right degree, always produce a large proportion oi 

 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 com- 

 munity, As with the varieties 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 condi- 

 tion of certain members of the community, 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 modifi- 

 cations- This process must have been repeated many times, 

 until that prodigious amount of difference between the 

 fertile ard sterile females of the same species nas been 

 producer which we see in many social instincts. 



But we have not as yet touched on the acme of the dif- 

 ficulty ; 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 wonderful 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 abdomen 

 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 Eu. opean ants guard and imprison. 



It will indeed be thought that I have an overweening 

 confidence in the principle of natural selection, when I do 

 not admit that such wonderful and well-established facts 

 at once annihilate the theory. In the simpler case of 

 neuter insects all of one caste, which, as I believe, have 

 been rendered diff erent from the fertile males and females 



