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 : "I 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 



