108 The Galls of Essex ; a Contribution to a
structures and their contained protoplasm is the formation of
the gall or other morbid product. The whole process may be
compared with the local consequences of the insertion of
vaccine lymph, or any such morbid poison, in ourselves or
other animals. I say the local consequences, for we have no
clear evidence of what might be called general infection or
constitutional disease in the gall-forming plants. In the
absence of quickly moving fluid, such as lymph or blood, the
virus infects only the part in and very near to which it is
inserted. A single oak-leaf may have fifty 'spangle' galls
on its under-surface, but the structures between them may be,
quite healthy ; and when in any instance a general damage
is done to a plant by gall-growths, it seems to be only as a
remote consequence of the spoiling of considerable portions
of its structures. And this appears to be true, even though
the virus may continue active for a long time, as in the galls
which begin to grow soon after the insertion of the virus
with the ovum, and continue to increase during the whole—
sometimes long—development of the larvae; or even, in a few
instances, after the larvae have deserted them.
"We find hundreds of different forms of galls, and we may
be nearly sure that there are as many kinds of morbid
poisons produced by the gall-insects, each form answering to
a different virus. This may suggest that we may be too
grudging in thinking of the number of morbid poisons, or of
their modifications in the blood, to which diseases in ourselves
may, at least in part, be due. 
"It is true that the galls are produced by many species of
insects on many species of plants; and that the differences
among these species may be as wide as those between
ourselves and any other Mammalia. But, even among
closely allied species, there are many and very different
forms of galls. Mayr, ten years ago, described and figured
ninety-six kinds of galls found on the oaks of Central Europe,
all but two of them being produced by different species of
gall-wasps. Of those ninety-six kinds, thirty-two are formed
on the leaves alone; and even on similar parts of one oak-leaf
it is not rare to find three or four different forms of galls.
"We have, thus, clear evidence of a very large number of
morbid poisons, each of which is capable of producing, in an
appropriate subject, a distinct specific disease with a charac-
teristic morbid structure.
* * * * * * *