110 The Galls of Essex ; a Contribution to a
"Very rarely, the same insect may produce on one leaf
different forms of galls.
"In all these points you may, I think, find help in the study
of specific diseases. . I will add only one more. Usually, the
gall begins to grow directly after the deposit of the egg; but
sometimes there is a long delay, a long period of suspense, an
"Eiruhe," which may last for many months before the growth
begins. What is going on during this time ? I believe we
may see here an instance of events very difficult to study in
our own pathology, in which two or more conditions must
concur to the production of some disease, and one of them
must wait for the complete efficiency of the rest. In the case
of these long-delayed galls, either the egg, after being laid,
requires a long time for the completion of changes ending in
the production of the necessary morbid poison, or the plant-
structure in which it is laid requires the time for changes to
make it susceptible of the poison; or both egg and plant may
need to change. So, in us, two or more conditions must
concur. A tendency to gout may be inherited, and the blood
may have slowly acquired the necessary morbid condition;
but no structure may be susceptible of gouty disease till a
blow, or a strain, or some disturbance of nervous force makes
it so. So with cancer; a general tendency may be inherited,
but it must wait till the material of some structure is, by age,
or injury, or long-continued 'irritation,' changed into fitness
for concurrence in morbid action with the material on which
the general tendency depends. * * * * In the growth
of these galls, the comparison may seem less far-fetched. At
least, it may be difficult to suggest any nearer comparison
for a process in which the meeting of two living materials
from different organisms is immediately followed by such a
change in the method of life of one of them, as ends in the
production of a definite new growth exactly adapted to the
method and purpose of the life of the other.
"But it is more than time that I should have done with
galls. If I have been tedious, let me assure you that I am
myself ashamed to have gathered so little from the rapidly
increasing records concerning them to which the botanists,
and still more the entomologists, of our time are contributing.
And, even for that little, I feel as if I deserved to be
compared with one of those burglars of whom I spoke as
feeding on the results of other's labours. Let it be my
apology, that I believe I have taken nothing that those others
would have used. I have only taken from their rich stores of
facts some that may be much more useful in pathology than