236 
INSTINCT. 
Chap. VII. 
hardly have been acted on by natural selection; cases of 
instincts almost identically the same in animals so re¬ 
mote in the scale of nature, that we cannot account 
for their similarity by inheritance from a common 
parent, and must therefore believe that they have 
been acquired by independent acts of natural selection. 
I will not here enter on these several cases, but will 
confine myself to one special difficulty, which at first 
appeared to me insuperable, and actually fatal to my 
whole theory. I allude to the neuters or sterile females 
in insect-communities: for these neuters often differ 
widely in instinct and in structure from both the males 
and fertile females, and yet, from 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 
structure; 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 community that a number 
should have been annually born capable of work, but in¬ 
capable of procreation, I can see no very great difficulty 
in this being effected by 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 some¬ 
times of eyes, and in instinct. As far as instinct alone 
is concerned, the prodigious difference in this respect 
between the workers and the perfect females, would 
have been far better exemplified by the hive-bee. If a 
working ant or other neuter insect had been an animal 
