List of the Insect Fauna of the Comity. 107
insect selects, by instinct, the very part of the plant which
is adapted to its purpose. The provision made at the cost of
the plant is exactly adapted in quality to the welfare of the
insect or its offspring, and in quantity as well; for both the
lodging and the food are made sufficient for any necessary
time—for days or weeks, often for many months; in some
instances for two or even three years. Nay, more than this,
a gall, of which the growth has been provoked by the virus
of one insect, may be fit for the food and lodging of another,
which, when all seems complete, can penetrate the gall-
cavity, and there, as with theft or murder, obtain food and
lodging perfectly suited to itself or its progeny. And the
whole process in the plant, though it be one of disease, and,
in a sense, unnatural, is yet so regular, so constant and
specific, that the form and other characters of each gall or
other morbid product are usually as constant and charac-
teristic as are those of the insect itself, and the differences
among the galls are at least as great as those among the
insects. Is there, in all the range of natural history, a more
marvellous group of facts than may here be studied ? If you
would like to work out a problem in evolution, find how it
has come to be a part of the ordinary economy of nature
that a gall-insect compels some part of a plant to grow in a
manner which, while injurious to the plant, becomes useful
to one insect not yet born, and to another who will in due
time invade the gall and kill and feed upon its occupant, and
then may itself be invaded and eaten by a third.
"But now of the relation between galls and our specific
diseases, such as our eruptive fevers, syphilis, cancer, gout,
and others.
" In these galls and other similar diseases in plants we have,
it seems, hundreds of specific diseases due to as many
hundreds of specific morbid poisons; for the most reasonable,
if not the only reasonable, theory of these diseases is, that
each insect infects or inoculates the leaf or other structure of
the chosen plant with a poison peculiar to itself. The poison
may be merely deposited ; but, in the instances best for study,
it is inserted in the plant-structure, whether leaf or any
other; and the wound for inserting it, the poisoned wound,
may be made either with part of the oral apparatus, or, as in
most of the true galls, with the ovipositor through which one
or more eggs are passed with the virus, and are left among
actively living structures of the plant. The little wound
closes; the virus, whether an oral or an ovarian secretion,
remains; and the result of its influence on the plant