102 The Galls of Essex; a Contribution to a
that surrounds it, becoming considerably larger before it is
hatched than it was when first deposited. When chemically
analysed, galls are found to contain only the same principles
as the plant from which they spring, but in a more con-
centrated state."
Hartig, in his specially scientific memoirs on the cynip-
ideous gall-makers, treats somewhat lengthily on the
physiology of the gall insects themselves, but on the growth
of their productions he says but little, limiting himself
strictly to the facts observed. He merely states, "The gall-
flies are divisible into three groups, according to their mode
of life. To the first of these belong those species which live
some on the plant-juices and some on the cellular tissue and
cork (starchy portions). These alone produce, through their
oviposition in certain plants and parts of plants, an hyper-
trophy of the part from the usual form, and shape the plant-
substance into a larva-house, the inner structure and outer
form of which is dependent on the producing species of gall-
fly. Every true gall-producing gall-fly has its own special
form of gall, which is constant within certain limits; and we
never find two similar species of true gall-fly inhabiting
different formed galls, nor do we breed from similarly formed
galls different species of gall-flies of the first group, which I
therefore call Psenides."9
To come to the more feasible theory, as expounded by
Dr. Hollis, we find that Van der Hoeven started on what may
be considered the right road, when, after speaking of the
deposition of the egg by the female Cynips, he says, "The
irritation thus produced causes the sap to flow in greater
abundance to the wounded part."10 Dr. Johnston, of
Berwick, marked a still clearer line when he tells us, "We
observe that the irritation caused by the deposition and
evolution of the egg will produce growth of the most curious
kind, and differences in the irritation, too slight to be traced,
will occasion very remarkable differences in the appearance of
9 Translated from Germar's 'Zeitschrift fur die Entomologie,' vol. iii.
p. 322. Leipzig, 1841.
10 'Handbook of Zoology,' translated by Clark, 18-56, i. 384.