knowledge of the historical transformation of the latter.

* * * * *

What, now, is life? To most people "life seems to be something which
enters into a combination of carbon and hydrogen and the other elements,
and makes this complex substance, the protoplasm, perform its various
activities." Nearly every one finds it difficult to regard life and
vitality as anything but actuating principles that exist apart from the
materials into which they enter, and which they seem to make alive.
According to this general conception, "life is something like an engineer
who climbs into the cab of the locomotive and pulls the levers which make
it go," as health might supposedly be regarded as something that does not
inhere in well-being, but gets into the body to alter it. But is this
conception really justified by the facts of animal structure and
physiology? Let us recall the steps of our analysis. The living organism
is a collection of differentiated parts, the organs; the life of an
organism is a series of activities of the several organic systems and
organs. If we could take away one organ after another, there would be
nothing left after the last part had been subtracted. In a similar manner,
the activities of organs prove to be the combined activities of the
tissue-cells, and again the truth of this statement will be clear when we
imagine the result of taking away one cell after another from organisms
like the frog or tree. When the last cell had been withdrawn, there would
be nothing left of the frog's structure, and there would be no element of
the frog's life. It is true that the particular way the tissue-cells are
combined is of primary importance, but it is none the less true that the
life of a cell is the kind of element out of which the life of even the
most complex organism is built. And we have seen that the essential
substance of a cell is a complex chemical compound we call protoplasm,