Chap. VIL 
NEUTEK INSECTS. 
239 
different: in Cryptocerus, the workers of one caste alone 
carry a wonderful sort of shield on their heads, the use 
of which is quite unknown: in the Mexican Myrme- 
cocystus, the workers of one caste never leave the nest; 
they are fed by the workers of another caste, and they 
have an enormously developed abdomen which secretes 
a sort of honey, supplying the place of that excreted by 
the aphides, or the domestic cattle as they may be called, 
which our European ants guard or imprison. 
It will indeed be thought that I have an overweening 
confidence in the principle of natural selection, when I 
do not admit that such wonderful and well-established 
facts at once annihilate my theory. In the simpler 
case of neuter insects all of one caste or of the same 
kind, which have been rendered by natural selection, as 
I believe to be quite possible, different from the fertile 
males and females,—in this case, we may safely conclude 
from the analogy of ordinary variations, that each suc¬ 
cessive, slight, profitable modification did not probably 
at first appear in all the individual neuters in the same 
nest, but in a few alone ; and that by the long-continued 
selection of the fertile parents which produced most 
neuters with the profitable modification, all the neuters 
ultimately came to have the desired character. On this 
view we ought occasionally to find neuter-insects of the 
same species, in the same nest, presenting gradations of 
structure; and this we do find, even often, considering 
how few neuter-insects out of Europe have been carefully 
examined. Mr. F. Smith has shown how surprisingly 
the neuters of several British ants differ from each other 
in size and sometimes in colour; and that the extreme 
forms can sometimes be perfectly linked together by 
individuals taken out of the same nest: I have myself 
compared perfect gradations of this kind. It often 
happens that the larger or the smaller sized workers 
