Darwin's Artificial and Natural Selection 143 
occasionally become sterile; and if such insects had been 
social, and it had been profitable to the community 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 in 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 unhesitat- 
ingly assumed that all its characters had been slowly ac- 
quired 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 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?” 
Darwin’s answer is that the differences of structure are 
correlated with certain ages and with the two sexes, but this 
is obviously only shifting the difficulty, not meeting it. He 
concludes, “I can see no great difficulty in any character 
becoming correlated with the sterile condition of certain 
members of the insect communities, 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 lessened, or, 
as I believe, disappears, when it is remembered that selection 
