eceiving from it its food and oxygen instead of taking it from the
water.

But in another sense the cells of our body live an entirely
different life, for they form a community. Division of labor has
taken place between them, they are interdependent, correlated with
one another, subject therefore to the laws of the whole community or
organism. There are many respects in which it is impossible to
compare Robinson Crusoe with a workman in a huge watch factory; yet
they are both men.

Both the amoeba and we live in the closest relation to our
environment, and conformity to it is evidently necessary: life has
been defined as the adjustment of internal relations to external
conditions. We continually take food, use it for energy and growth,
and return the simpler waste compounds. We are all of us, as
Professor Huxley has said, "whirlpools on the surface of Nature;"
when the whirl of exchange of particles ceases we die. We have seen
that the fusion of two amoebae results in a new rejuvenated
individual. Why is a mixture of two protoplasms better than one? We
can frame hypotheses; we know nothing about it. What of the mind of
the amoeba? A host of questions throng upon us and we can answer
no one of them. All the great questions concerning life confront us
here in the lowest term of the animal series, and appear as
insoluble as in the highest.

Our second ancestral form is also a fresh-water animal, the hydra.
This is a little, vase-shaped animal, which usually lives attached
to grass-stems or sticks, but has the power to free itself and hang
on the surface of the water or to slowly creep on the bottom. The
mouth is at the top of the vase, and the simple, undivided cavity
within the vase is the digestive cavity. Around the mouth is a ring
of from four to ten hollow tentacles, whose cavities communicate
freely underneath with the digestive cavity. Not only is food taken
in at the mouth, but indigestible material is thrown out here. The
animal may thus be compared to a nearly cylindrical sack with a
circle of tubes attached to it above. The body consists of two
layers of cells, the ectoderm on the outside and the entoderm lining
the digestive cavity. Between these two is a structureless, elastic
membrane, which tends to keep the body moderately expanded.

The food is captured by the tentacles; but digestion takes place
only partially in the digestive cavity, for each surrounding cell
engulfs small particles of food and dig