the reproduction of all animals and plants. We
can not, therefore, find any explanation of reproduction until we have
explained the mechanism of the cell. The fundamental feature, of
nature's machine building is thus based upon the machinery of the
nucleus and centrosome of the organic cell.

Aside from the simple fact that it preserves the race, the most
important feature connected with this reproduction is its wonderful
fruitfulness. Since it results from division, it always tends to
increase the offspring in geometrical ratio. In the simplest case, that
of the unicellular animals, the cell divides, giving rise to two
animals, each of which divides again, producing four, and these again,
giving eight, etc. The rapidity of this multiplication is sometimes
inconceivable. It depends, of course, upon the interval of time between
the successive divisions, but among the lower organisms this interval is
sometimes not more than half an hour, the result of which is that a
single individual could give rise in the course of twenty-four hours to
sixteen million offspring. This is doubtless an extreme case, but among
all the lower animals the rate is very great. Among larger animals the
process is more complicated; but here, too, there is the same tendency
to geometrical progression, although the intervals between the
successive reproductions may be quite long and irregular. But it is
always so great that if allowed to progress unhindered at its normal
rate the offspring would, in a few years, become so numerous as to crowd
other life out of existence. Even the slow-breeding elephant would, if
allowed to breed unhindered for seven hundred and fifty years, produce
nineteen million offspring--a rate of increase plainly incompatible with
the continued existence of other animals.

Here, then, we have the foundation of nature's method of building
animals and plants of the higher classes. In the machinery of the cell
she has a power of reproduction which produces an increase in
geometrical ratio far beyond the possibility for the surface of the
earth to maintain.

==Heredity.==--The offspring which arise by these processes of division
are like each other, and like the parent from which they sprung. This
is the essence of what is called heredity. Its significance in the
process of machine building is evident at once. It is the conserving
force which preserves the forms already produced and makes it possible
for each generation to bui