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 receiving 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.