292 Darwin, and after Darwin. 
know any of his followers who have made any ap- 
proach to an adequate use of it in their advocacy 
of his views. In preparing the present chapter, 
therefore, I have been particularly careful not to pitch 
too high my own estimate of its evidential value. 
That is to say, I have considered, both in the domain 
of structures and of instincts, what instances admit of 
being possibly adduced per contra, or as standing out- 
side the general law that adaptive structures and 
instincts are of primary use only to their possessors. 
In the result I can only think of two such instances. 
These, therefore, I will now dispose of. 
The first was pointed out, and has been fully dis- 
cussed, by Darwin himself. Certain species of ants 
are fond of a sweet fluid that is secreted by aphides, 
and they even keep the aphides as we keep cows for 
the purpose of profiting by their “milk.’ Now the 
point is, that the use of this sweet secretion to the 
aphis itself has not yet been made out. Of course, if 
it is of no use to the aphis, it would furnish a case 
which completely meets Darwin’s own challenge. But, 
even if this supposition did not stand out of analogy 
with all the other facts of organic nature, most of us 
would probably deem it prudent to hold that the 
secretion must primarily be of some use to the aphis 
itself, although the matter has not been sufficiently 
investigated to inform us of what this use is. For, in 
any case, the secretion is not of any vital importance 
to the ants which feed upon it: and I think but few 
impartial minds would go so far to save an hypothesis 
as to maintain, that the Divinity had imposed this drain 
upon the internal resources of one species of insect 
for the sole purpose of supplying a luxury to another. 
