into compounds of a more complex structure, and forms an integral
part of the animal body. The dead food particle has been transformed
into living protoplasm, the continually repeated miracle of life.
But it does not remain long in this condition. In contact with the
oxygen from the air it is soon oxidized, burned up to furnish the
energy necessary for the motion and irritability of the body. We are
all of us low-temperature engines. The digestive function exists in
all animals merely to bring the food into a soluble, diffusible
form, so that it can pass to all parts of the body and be used for
fuel or growth. In our body a circulatory system is necessary to
carry food and oxygen to the cells and to remove their waste. For
most of our cells lie at a distance from the stomach, lungs, and
kidney. But in a small animal the circulatory system is often
unnecessary and fails. Breathing and excretion take place through
the whole surface of the body. The body of the frog is devoid of
scales, so that the blood is separated from the surrounding water
only by a thin membrane, and it breathes and excretes to a certain
extent in the same way.

But another factor has to be considered. If we double each dimension
of our amoeba, we shall increase its surface four times, its mass
eight-fold. Now the power of absorbing oxygen and excreting waste is
evidently proportional to the excretory and respiratory surface, and
much the same is true of digestion. But the amount of oxygen
required, and of waste to be removed is proportional to the mass;
for every particle of protoplasm requires food and oxygen, and
produces waste. The particles of protoplasm in our new, larger
amoeba can therefore receive only half as much oxygen as before,
and rid themselves of their waste only half as fast. There is
danger of what in our bodies would be called suffocation and
blood-poisoning. The amoeba having attained a certain size meets
this emergency by dividing into two small individuals, the division
is a physical adaptation. But the many-celled animal cannot do this;
it must keep its cells together. It gains the additional surface by
folding and plaiting. And the complicated internal structure of
higher animals is in its last analysis such a folding and plaiting
in order to maintain the proper ratio between the exposed surface of
the cells and their mass. And each cell in our bodies lives in one
sense its own individual life, only bathed in the lymph and
r