List of the Insect Fauna of the County. 109
"It may be observed, however, that these great differences
are marked in outer shape and construction much more than
in minute structure. As, in human pathology, there are
certain general characters and degrees of likeness in all
inflammatory products, however differently they may be
constructed—in pustules, vesicles, thickenings, opacities,
adhesions, scars, fibroid and other changes; so, in galls,
there are certain likenesses in minute structures, even among
those that are, in their construction, size, and outer shape,
most unlike.
"It may be well to learn from this a lesson on the imper-
fection of our methods of minute research. As we cannot
doubt that the differences in outer shape and method of
construction of the producta of specific diseases are associated
with differences of chemical composition and ultimately
minute structure, so it must be in those yet greater differences
on which we frame our distinctions of species in all living
nature. The coarse, visible, and tangible distinctions may
be well marked; the really material differences with which
these are associated, and to which probably they are due, are
beyond our reach.
" Again, in the study of specific diseases in ourselves, we see
many variations due to the differences in the parts, or
even in the persons affected with them. In the study of
galls, similar variations may be seen. As a general rule,
each gall-insect lays its eggs in one part of one plant—as the
leaf, leaf-stalk, bud, fruit, or root of this or that species; but
if—as rarely happens—one lays in different parts of the same
plant, there is usually a very close agreement in the characters
of the resultant galls. A few exceptions to this rule are
known, one of them being in the very different galls produced
on the roots and on the leaves of vines by Phylloxera vastatrix;
but the rule is generally observed, and accords with the fact
of certain features of general likeness being observed in the
products of our several specific diseases, wherever they may
be seated.
"When the same insect lays in similar parts of different
plants, the galls may be all similar ; but I believe that they
more usually are different, and that their differences are such
as bring them severally nearer to the distinctive characters
of the plants on which they grow; just as, in ourselves, a
specific disease may be modified by the personal conditions of
each patient.
"In similar analogy, the differences are yet greater when
the eggs are laid in different parts of different plants.