INSTINCT 885 



selected, and so onward. But with the working ant we 

 have an insect differing greatly from its parents, yet 

 absolutely sterile; so that it could never have trans- 

 mitted successively acquired modifications of structure 

 or instinct to its progeny. It may well be asked how 

 is it possible to reconcile this case with the theory of 

 natural selection ? 



First, let it be remembered that we have innumerable 

 instances, both in our domestic productions and in those 

 in a state of nature, of all sorts of differences of inherited 

 structure which are correlated with certain ages, and with 

 either sex. We have differences correlated not only with 

 one sex, but with that short period when the reproduc- 

 tive system is active, as in the nuptial plumage of many 

 birds, and in the hooked jaws of the male salmon. We 

 have even slight differences in the horns of different 

 breeds of cattle in relation to an artificially imperfect 

 state of the male sex; for oxen of certain breeds have 

 longer horns than the oxen of other breeds, relatively 

 to the length of the horns in both the bulls and cows 

 of these same breeds. Hence I can see no great diffi- 

 culty in any character becoming correlated with the 

 sterile condition of certain members of insect-commu- 

 nities: the difficulty lies in understanding how such 

 correlated modifications of structure could have been 

 slowly accumulated by natural selection. 



This difficulty, though appearing insuperable, is less- 

 ened, or, as I believe, disappears, when it is remem- 

 bered 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 characterized has 



—Science — 17 



