nd, on the one hand, that "roots
can become directly transformed into leaf-bearing shoots," and, on the
other hand, that in some plants certain "apparent roots are only
underground shoots," and that nevertheless "they are similar to true
roots in function and in the formation of tissue, but have no root-cap,
and, when they come to the light above ground, continue to grow in the
manner of ordinary leaf-shoots."[56] If, then, in highly developed
plants inheriting pronounced structures, this differentiating influence
of the medium is so marked, it must have been all-important at the
outset while types were undetermined.

As with plants so with animals, we find good reason for inferring that
while the specialities of the tegumentary parts must be ascribed to the
natural selection of favourable variations, their most general traits
are due to the direct action of surrounding agencies. Here we come upon
the border of those changes which are ascribable to use and disuse. But
from this class of changes we may fitly exclude those in which the parts
concerned are wholly or mainly passive. A corn and a blister will
conveniently serve to illustrate the way in which certain outer actions
initiate in the superficial tissues, effects of very marked kinds, which
are related neither to the needs of the organism nor to its normal
structure. They are neither adaptive changes nor changes towards
completion of the type. After noting them we may pass to allied, but
still more instructive, changes. Continuous pressure on any portion of
the surface causes absorption, while intermittent pressure causes
growth: the one impeding circulation and the passage of plasma from the
capillaries into the tissues, and the other aiding both. There are yet
further mechanically-produced effects. That the general character of the
ribbed skin on the under surfaces of the feet and insides of the hands
is directly due to friction and intermittent pressure, we have the
proofs:--first, that the tracts most exposed to rough usage are the most
ribbed; second, that the insides of hands subject to unusual amounts of
rough usage, as those of sailors, are strongly ribbed all over; and
third, that in hands which are very little used, the parts commonly
ribbed become quite smooth. These several kinds of evidence, however,
full of meaning as they are, I give simply to prepare the way for
evidence of a much more conclusive kind.

Where a wide ulcer has eaten away the deep-s