know every material living thing is composed wholly of
protoplasm and of the structures which it has built up.

This grayish, viscid, slimy, semi-transparent, semi-fluid substance,
similar to the white of an egg, is the most puzzling, the most wonderful
material with which science has to deal. Chemically it is composed of
various proteids, fats, carbohydrates, etc., and these in turn of but
very few elements, all of which are common, and none of which are
peculiar to protoplasm itself. And yet its essential properties, its
mechanical as well as its chemical make-up, have baffled the resources
of our wisest men with all their retorts and microscopes and other
instruments of precision.

Protoplasm is essentially uniform and similar in appearance and
properties wherever found, whether in the tissues of the human body, in
a blade of grass, or in the green slime of a stagnant pool. And yet
probably no two samples of protoplasm are ever exactly similar in all
respects, though we may never be able to detect their precise
differences. These differences are due to the fact that the stuff is
_alive_, and within it are constantly going on those changes
accompanying metabolism, or the building up and tearing down processes
that always accompany life. All separate masses of protoplasm, such as
the one-celled amoeba or the individual cells of our own bodies, are
constantly taking in food and as constantly throwing off wastes. Hence,
in the very nature of things, it is impossible to find any mass of
protoplasm absolutely pure. And a further and impassable barrier to
chemical analysis, or indeed to any adequate scientific examination,
lies in the fact that we can never deal with protoplasm exactly as it
is, since no analysis can be performed upon it without destroying its
life. And yet even dead protoplasm, and especially its most
characteristic constituent, _proteid_, has been found the most difficult
material in the world to analyze, and nobody as yet pretends to know its
exact chemical make-up.

The constant effort of natural science to press back the boundaries of
the unknown is very liable to obscure some of the things most essential
to any system of clear thinking regarding these matters. We are so prone
to think that if only our microscopes were a little stronger, if only we
could devise more effective methods of staining or of chemical analysis
or chemical synthesis, we might really find out what life is, or what
matter its