peculiar kinds of
nutriment, independently of their union with distinct gemmules. We shall
appreciate this difficulty if we call to mind, what complex yet symmetrical
growths the cells of plants yield when they are inoculated by the poison of
a gall-insect. With animals various polypoid excrescences and tumours are
now generally admitted[910] to be the direct product, through
proliferation, of normal cells which have become abnormal. In the regular
growth and repair of bones, the tissues undergo, as Virchow remarks,[911] a
whole series of permutations and substitutions. "The cartilage-cells may be
{382} converted by a direct transformation into marrow-cells, and continue
as such; or they may first be converted into osseous and then into
medullary tissue; or lastly, they may first be converted into marrow and
then into bone. So variable are the permutations of these tissues, in
themselves so nearly allied, and yet in their external appearance so
completely distinct." But as these tissues thus change their nature at any
age, without any obvious change in their nutrition, we must suppose in
accordance with our hypothesis that gemmules derived from one kind of
tissue combine with the cells of another kind, and cause the successive
modifications.

It is useless to speculate at what period of development each organic unit
casts off its gemmules; for the whole subject of the development of the
various elemental tissues is as yet involved in much doubt. Some
physiologists, for instance, maintain that muscle or nerve-fibres are
developed from cells, which are afterwards nourished by their own proper
powers of absorption; whilst other physiologists deny their cellular
origin; and Beale maintains that such fibres are renovated exclusively by
the conversion of fresh germinal matter (that is the so-called nuclei) into
"formed material." However this may be, it appears probable that all
external agencies, such as changed nutrition, increased use or disuse, &c.,
which induced any permanent modification in a structure, would at the same
time or previously act on the cells, nuclei, germinal or formative matter,
from which the structures in question were developed, and consequently
would act on the gemmules or cast-off atoms.

There is another point on which it is useless to speculate, namely, whether
all gemmules are free and separate, or whether some are from the first
united into small aggregates. A feather, for instance, is a comple