Chap. VIII.] OF NATURAL SELECTION". 229 



from both the males and fertile females, and yet, irom being sterile, 

 they cannot propagate their kind. 



The subject well deserves to be discussed at great length., but I 

 will here take only a single case, that of working or sterile ants. 

 How the workers have been rendered sterile is a difficulty ; but not 

 much greater than that of any other striking modification of struc- 

 ture ; for it can be shown that some insects and other articulate 

 animals in a state of nature occasionally become sterile ; and if 

 such insects had been social, and it had been profitable to the com- 

 munity that a number should have been annually born capable of 

 work, but incapable of procreation, I can see no especial difficulty 

 in this having been effected through natural selection. But I must 

 pass over this preliminary difficulty. The great difficulty lies in 

 the working ants differing widely from both the males and the 

 fertile females in structure, as in the shape of the thorax, and iix 

 being destitute of wings and sometimes of eyes, and in instinct. 

 As far as instinct alone is concerned, the wonderful difference in 

 this respect between the workers and the perfect females, would 

 have been better exemplified by the hive-bee. If a working ant or 

 other neuter insect had been an ordinary animal, I should have 

 unhesitatingly assumed that all its characters had been slowly 

 acquired through natural selection ; namely, by individuals having 

 been born with slight profitable modifications, which were inherited 

 by the offspring; and that these again varied and again were 

 selected, and so onwards. But with the working ant we have an 

 insect differing greatly from its parents, yet absolutely sterile ; so 

 that it could never have transmitted successively acquired modifica- 

 tions 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 corre- 

 lated not only with one sex, but with that short period when the 

 reproductive 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 rela- 

 tion to an artificially imperfect state of the male sex; for oxen 01 

 certain breeds have longer horns than the oxen of other breeds, 

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

 these same breeds. Hence I can see no great difficulty in any 

 character becoming correlated with the sterile condition of certaia 



