to note
that in those aggregates of cells constituting the _Metaphyta_ and
_Metazoa_, analogous distributions also exist. In plants they are of
course not to be looked for in leaves and other deciduous portions, but
only in portions of long duration--stems and branches. Naturally, too,
we need not expect them in plants having modes of growth which early
produce an outer practically dead part, that effectually shields the
inner actively living part of the stem from the influence of the
medium--long-lived acrogens such as tree-ferns and long-lived endogens
such as palms. But in the highest plants, exogens, which have the
actively living part of their stems within reach of environing agencies,
we find this part,--the cambium layer,--is one from which there is a
growth inwards forming wood, and a growth outwards forming bark: there
is an increasingly thick covering (where it does not scale off) of
tissue changed by the medium, and inside this a film of highest
vitality. In so far as concerns the present argument, it is the same
with the _Metazoa_, or at least all of them which have developed
organizations. The outer skin grows up from a limiting plane, or layer,
a little distance below the surface--a place of predominant vital
activity. Here perpetually arise new cells, which, as they develop, are
thrust outwards and form the epidermis: flattening and drying up as they
approach the surface, whence, having for a time served to shield the
parts below, they finally scale off and leave younger ones to take their
places. This still undifferentiated tissue forming the base of the
epidermis, and existing also as a source of renewal in internal organs,
is the essentially living substance; and facts above given imply that it
was the action of the medium on this essentially living substance,
which, during early stages in the organization of the _Metazoa_,
initiated that protective envelope which presently became an inherited
structure--a structure which, though now mainly inherited, still
continues to be modifiable by its initiator.

Fully to perceive the way in which these evidences compel us to
recognize the influence of the medium as a primordial factor, we need
but conceive them as interpreted without it. Suppose, for instance, we
say that the structure of the epidermis is wholly determined by the
natural selection of favourable variations; what must be the position
taken in presence of the fact above named, that when mucous mem