in, the question will be answered by many with equal promptness in
the affirmative. At all events, we have learned in the last forty years
to recognize some of the factors which have been at work in the
construction of this machine. We must turn, therefore, to the
consideration of these factors.

==Forces at Work in the Building of the Living Machine.==--There are three
primary factors which lie at the bottom of the whole process. They are--

1. _Reproduction_, which preserves type from generation to generation.

2. _Variation_, which modifies type from generation to generation.

3. _Heredity_, which transmits characters from generation to generation.

Each must be considered by itself.

==Reproduction.==--Reproduction is the primary factor in this process of
machine building, heredity and variation being simply phases of
reproduction. The living machine has developed by natural processes, all
other machines by artificial methods. Reproduction is the one essential
point of difference between the living machine and the others which has
made their construction by natural processes a possibility. What, then,
is reproduction? Reproduction is in all cases at the bottom simple
division. Whether we consider the plant that multiplies by buds or the
unicellular animal that simply divides into two equal parts, or the
larger animal that multiplies by eggs, we find that in all cases the
fundamental feature of the process is division. In all cases the
organism divides into two or more parts, each of which becomes in time
like the original. Moreover, when we trace this division further we find
that in all cases it is to be referred back to the division of the cell,
such as we have described in a previous chapter. The egg is a single
cell which has come from the parent by the division of one of the cells
in the body of the parent. A bud is simply a mass of cells which have
all arisen from the parent cells by division. The foundation of
reproduction is thus in all cases cell division. Now, this process of
division is dependent upon the properties of the cell. Firstly, it is a
result of the assimilative powers of the cell, for only through
assimilation can the cell increase in size, and only as it increases in
size can it gain sustenance for cell division. Secondly, it is
dependent, as we have seen, upon the mechanism of the cell body, and
especially the nucleus and centrosome. These structures regulate the
cell division, and hence