It is a little mass of protoplasm containing a nucleus, and corresponds, therefore, to one of the cells, most closely to the egg-cell or spermatozoon of higher animals.  If every living being is descended from a single cell, the fertilized egg, it is not hard to believe that all higher animals are descended from an ancestor having the general structure or lack of structure of the amoeba.

But is the amoeba really structureless?  Probably it has an exceedingly complex structure, but our microscopes and technique are still too imperfect to show more than traces of it.  Says Hertwig:  “Protoplasm is not a single chemical substance, however complicated, but a mixture of many substances, which we must picture to ourselves as finest particles united in a wonderfully complicated structure.”  Truly protoplasm is, to borrow Mephistopheles’ expression concerning blood, a “quite peculiar juice.”  And the complexity of the nucleus is far more evident than that of the protoplasm.  Is protoplasm itself the result of a long development?  If so, out of what and how did it develop?  We cannot even guess.  But the beginning of life may, apparently must, have been indefinitely farther back than the simplest now existing form.  The study of the amoeba cannot fail to raise a host of questions in the mind of any thoughtful man.

As we have here the animal reduced, so to speak, to lowest terms, it may be well to examine a little more closely into its physiology and compare it briefly with our own.

The amoeba eats food as we do, but the food is digested directly in the internal protoplasm instead of in a stomach; and once digested it diffuses to all parts of the cell; here it is built up 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.