ginal units--largely as survival of
the fittest has been instrumental in furthering and controlling the
combination of these units into visible organisms, and eventually into
large ones; yet we must ascribe to the direct effect of the medium
on the first forms of life, that character of which this
everywhere-operative factor has taken advantage.

* * * * *

Let us turn now to another and more obvious attribute of higher
organisms, for which also there is this same general cause. Let us
observe how, on a higher platform, there recurs this differentiation of
outer from inner--how this primary trait in the living units with which
life commences, re-appears as a primary trait in those aggregates of
such units which constitute visible organisms.

In its simplest and most unmistakable form, we see this in the early
changes of an unfolding ovum of primitive type. The original fertilized
single cell, having by spontaneous fission multiplied into a cluster of
such cells, there begins to show itself a contrast between periphery and
centre; and presently there is formed a sphere consisting of a
superficial layer unlike its contents. The first change, then, is the
rise of a difference between that outer part which holds direct converse
with the surrounding medium, and that inclosed part which does not. This
primary differentiation in these compound embryos of higher animals,
parallels the primary differentiation undergone by the simplest living
things.

Leaving, for the present, succeeding changes of the compound embryo, the
significance of which we shall have to consider by-and-by, let us pass
now to the adult forms of visible plants and animals. In them we find
cardinal traits which, after what we have seen above, will further
impress us with the importance of the effects wrought on the organism by
its medium.

From the thallus of a sea-weed up to the leaf of a highly developed
phaenogam, we find, at all stages, a contrast between the inner and
outer parts of these flattened masses of tissue. In the higher _Algae_
"the outermost layers consist of smaller and firmer cells, while the
inner cells are often very large, and sometimes extremely long;"[51] and
in the leaves of trees the epidermal layer, besides differing in the
sizes and shapes of its component cells from the parenchyma forming the
inner substance of the leaf, is itself differentiated by having a
continuous cuticle, and by having the