Darwin's Artificial and Natural Selection 145 
thus characterized. According to this view we ought occa- 
sionally to find in the same nest neuter insects, presenting 
gradations of structure; and this we do find, even not rarely, 
considering how few neuter insects out of Europe have been 
carefully examined.” 
From this the conclusion is reached : — 
“With these facts before me, I believe that natural selec- 
tion, by acting on the fertile ants or parents, could form a 
species which should regularly produce neuters, all of large 
size with one form of jaw, or all of small size with widely dif- 
ferent jaws; or lastly, and this is the greatest difficulty, one 
set of workers of one size and structure, and simultaneously 
another set of workers of a different size and structure ; —a 
graduated series having first been formed, as in the case of 
the driver ant, and then the extreme forms having been pro- 
duced in greater and greater numbers, through the survival 
of the parents which generated them, until none with an 
intermediate structure were produced. 
“T have now explained how, as I believe, the wonderful 
fact of two distinctly defined castes of sterile workers exist- 
ing in the same nest, both widely different from each other 
and from their parents, has originated. We can see how 
useful their production may have been to a social community 
of ants, on the same principle that the division of labor is 
useful to civilized man. Ants, however, work by inherited 
instincts and by inherited organs or tools, whilst man works 
by acquired knowledge and manufactured instruments. But 
I must confess, that, with all my faith in natural selection, I 
should never have anticipated that this principle could have 
been efficient in so high a degree, had not the case of these 
neuter insects led me to this conclusion. I have, therefore, 
discussed this case, at some little but wholly insufficient 
length, in order to show the power of natural selection, and 
likewise because this is by far the most serious special diffi- 
culty which my theory has encountered. The case, also, is 
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