higher animals. And these unicellular animals have the rudiments of
all our functions. Their protoplasm and functions seem to differ
from those of higher animals only in degree, not in kind. And the
more we consider both these facts the more remarkable and suggestive
do they become.

Cells with membranes can unite in colonies capable of division of
labor and differentiation. And magosphaera is just such a little
spheroidal colony. But the cells are still all alike, each one
performs all functions equally well. But in volvox division of labor
and differentiation of structure have taken place. Certain cells
have become purely reproductive, while the rest gather nutriment for
these, but are at the same time sensitive and locomotive, excretory
and respiratory. The first function to have cells specially devoted
to it is the reproductive; this is a function absolutely necessary
for the maintenance of the species. For the nutritive cells die when
they have brought the reproductive cells to their full development.
These few nutritive cells represent the body of all higher animals
in contrast with the reproductive elements. And with the development
of a body, death, as a normal process, enters the world. The
dominant function is here evidently the reproductive, and the whole
body is subservient to this.

In hydra the union and differentiation of cells is carried further.
But the cells are still much alike and only slowly lose their own
individuality in that of the whole animal. This is shown in the fact
that each entodermal cell digests its own particles of food,
although the nutriment once digested diffuses to all parts of the
body. Also almost any part of the animal containing both ectoderm
and entoderm can be cut off and will develop into a new animal.

But beside the reproductive cells and tissues hydra has developed a
very simple digestive system, in which the newly caught food at
least macerates and begins to be dissolved. This is the second
essential function. The animal can, and the plant as a rule does,
exist with only the lowest rudiments of anything like nervous or
muscular power; but no species can exist without good powers of
digestion and reproduction. These essential organs must first
develop and the higher must wait. And the inner, digestive, layer of
cells persists in our bodies as the lining of the mid-intestine. We
compared hydra therefore to a little patch of the lining of our
intestine covered with a fl