gested food, can absorb oxygen, can use this oxygen in the burning
of its own substance to produce its own activities, can act in
response to sensation gained from outside, can throw off its waste
matter produced by its own activities, and can grow. When the proper
time comes its nucleus can split in two, the cell itself enclosing the
nucleus can separate into two cells, each of which can grow to the
size of the parent cell and repeat its life. This is as simple an
animal as we have yet discovered. Every kitchen drain swarms with such
creatures. On a summer day the stagnant pools are full of them. The
simplest microscope will show them clearly. This is life in its lowest
terms with which we are acquainted. With such life, it seems to us,
the animal and plant world must have started their existence, when
first the earth began to teem with living matter.

If, then, we may form any judgment concerning the first living things
upon the globe by considering the simplest creatures that live here
to-day, certain facts seem clear. In the first place, life began in
the water, and for a long time was only to be found in the water.
Single cells are so small and dry out so easily that it is necessary
to their existence that they should be kept entirely moist by the
presence of water all about them. It is true many of them will stand
drying, but while they are thus dried they can scarcely be said to be
much more than just alive. They are utterly inactive, or, as we say,
they are dormant. In such conditions they become covered with a tough
skin, almost a shell, and their protoplasm is itself nearly dry. Under
these circumstances the life processes hardly continue at all. The
protozoa, as these small animals are called, tolerate drought for a
time; but they only live, in any sense worth calling living, when
water is abundant and is neither very warm nor very cold. It is safe
to say that the early life of the world formed in the oceans of the
time. So absolutely is the habit fixed upon cells of protoplasm that
even to-day the activities of the cells of higher animals depend upon
the presence of moisture. The cells of our own bodies are to-day
living, as it were, in an ocean. Everyone can remember far enough back
to recall some time at which a tear slipped from his own eye onto his
own tongue; we know our tears are salt. The tongue has tasted,
undoubtedly, the perspiration from the lip on more than one summer
day; this perspiration taste