CHAP. VIII.] OF NATURAL SELECTION. 215 



understanding how such correlated modifications of structure 

 could have been slowly accumulated by natural selection. 



This difficulty, though appearing insuperable, is lessened, or, as 

 I believe, disappears, when it is remembered that selection may 

 be applied to the family, as well as to the individual, and may 

 thus gain the desired end. Breeders of cattle wish the flesh and 

 fat to be well marbled together : an animal thus characterised has 

 been slaughtered, but the breeder has gone with confidence to the 

 same stock and has succeeded. Such faith may be placed in the 

 power of selection, that a breed of cattle, always yielding oxen 

 with extraordinarily long horns, could, it is probable, be formed 

 by carefully watching which individual bulls and cows, when 

 matched, produced oxen 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 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 condition 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 modifications. This process must have been repeated 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 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 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 



