n 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 outside 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 discussed, 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. On the whole, it seems most probable that the fluid is of the
nature of an excretion, serving to carry off waste products. Such, at
all events, was the opinion at which Darwin himself arrived, as a result
of observing the facts anew, and in relation to his theory.

* * * * *

The other instance to which I have alluded as seeming at first sight
likely to answer Darwin's challenge is the formation of vegetable galls.
The great number and variety of galls agree in presenting a more or less
elaborate structure, which is not only foreign to any of the uses of
plant-life, but singularly and specially adapted to those of the
insect-life which they shelter. Yet they are produced by a growth of the
plant itself, when suitably stimulated by the insects' inoculation--or,
according to recent observations, by emanations from the bodies of the
larvae which develop