The Doctrine of Evolution | Page 9

in a liquid serum. Thus an organism proves to be a complex
mechanism composed of cells as structural units, just as a building is
ultimately a collection of bricks and girders and bolts, related to one
another in definite ways.
Our analysis reveals the living creature in an entirely new light, not
only as a machinelike structure whose parts are marvelously formed
and coordinated in material respects, but also as one whose activities or
workings are ultimately cellular in origin. Structure and function are
inseparable, and if an animal or a plant is an aggregate of cells, then its
whole varied life must be the sum total of the lives of its constituent
cells. Should these units be subtracted from an animal, one by one,
there would be no material organism left when the last cells had been
disassociated, and there would be no organic activity remaining when
the last individual cell-life was destroyed. All the various things we do
in the performance of our daily tasks are done by the combined action
of our muscle and nerve and other tissue cells; our life is all of their
lives, and nothing more. The cell, then, is the physiological or
functional unit, as truly as it is the material element of the organic
world. Being combined with countless others, specialized in various
ways, relations are established which are like those exhibited by the
human beings constituting a nation. In this case the life of the
community consists of the activities of the diverse human units that
make it up. The farmer, the manufacturer, the soldier, clerk, and artisan
do not all work in the same way; they undertake one or another of the
economic tasks which they may be best fitted by circumstances to
perform. Their differentiation and division of labor are identical with
the diversity in structure and in function as well, exhibited by the cells

of a living creature. We might speak of the several states as so many
organs of our own nation; the commercial or farming or manufacturing
communities of a state would be like the tissues forming an organ,
made up ultimately of human units, which, like cells, are engaged in
similar activities. As the individual human lives and the activities of
differentiated economic groups constitute the life of a nation and
national existence, so cell-lives make the living of an organism, and the
expressions "division of labor" and "differentiation" come to have a
biological meaning and application.
* * * * *
The cell, then, is in all respects the very unit of the organic world. Not
only is it the ultimate structural element of all the more familiar
animals and plants that we know, as the foregoing analysis
demonstrates, but, in the second place, the microscope reveals simple
little organisms, like Amoeba, the yeast plant and bacteria, which
consist throughout their lives of just one cell and nothing more. Still
more wonderful is the fact that the larger complex organisms actually
begin existence as single cells. In three ways, therefore,--the analytic,
the comparative, and the developmental,--the cell proves to be the
"organic individual of the first order." As the ultimate biological unit,
its essential nature must possess a profound interest, for in its substance
resides the secret of life.
This wonderful physical basis of life is called protoplasm. It contains
three kinds of chemical compounds known as the proteins,
carbohydrates, and hydrocarbons. Proteins are invariably present in
living cells, and are made up of carbon, hydrogen, nitrogen, sulphur,
and usually a little phosphorus. The elements are also combined in a
very complex chemical way. For example, the substance called
haemoglobin is the protein which exists in the red blood cells and
which causes those cells to appear light red or yellow when seen singly.
Its chemical formula states the precise number of atoms which enter
into the constitution of a single molecule as:
C_{600}H_{960}N_{154}FeO_{179}. This is truly a marvelously
complex substance when compared with the materials of the inorganic

world, like water, for example, which has the formula H_{2}O. And
just as the peculiar properties of H_{2}O are given to it by the
properties of the hydrogen and the oxygen which combine to form it,
just so, the scientist believes, the marvelous properties of protein are
due to the assemblage of the properties of the carbon and hydrogen and
other elements which enter into its composition.
It would be interesting to see how each one of these elements
contributes some particular characteristic to the whole compound. The
carbon atom, for example, is prone to combine with other atoms in
definite varied ways, and the high degree of complexity which the
protein molecule possesses may depend in greater part upon the
combining power of its carbon elements. The nitrogen atom makes the
protein an extremely volatile compound,