142 Evolution and Adaptation 
countries without slaves? Is the question so simple as this? 
May not the degeneration of the masters more than compen- 
sate for the acquirement of slaves, and may not the loss 
of life in obtaining slaves more than counterbalance the ad- 
vantage of the slaves after they are captured? In the face 
of these possibilities it is not surprising to find that Darwin, 
when summing up the chapter, makes the following admis- 
sion: “JI do not pretend that the facts in this chapter 
strengthen in any degree my theory; but none of .the cases 
of difficulty, to the best of my judgment, annihilate it.” 
Darwin, with his usual frankness, adds : — 
“No doubt many instincts of very difficult explanation 
could be opposed to the theory of natural selection, — cases, 
in which we cannot see how an instinct could have originated ; 
cases, in which no intermediate gradations are known to 
exist ; cases of instincts of such trifling importance, that they 
could hardly have been acted on by natural selection; cases 
of instincts almost identically the same in animals so remote 
in the scale of nature, that we cannot account for their 
similarity by inheritance from a common progenitor, and 
consequently must believe that they were independently 
acquired through 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 the 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 
