le advance. After a time some of the cells failed to
separate after division, but remained clinging together (Fig. 45). The
cells of such a mass must have been at first all alike; but, after a
little, differences began to appear among them. Those on the outside of
the mass were differently affected by their surroundings from those in
the interior, and soon the cells began to share among themselves the
different duties of life. The cells on the outside were better situated
for protection and capturing food, while those on the inside could not
readily seize food for themselves, and took upon themselves the duty of
digesting the food which was handed to them by the outer cells. Each of
these sets of cells could now carry on its own special duties to better
advantage, since it was freed from other duties, and thus the whole mass
of cells was better served than when each cell tried to do everything
for itself. This was the first step in the building of the machine out
of the active cells (Fig. 46). From such a starting point the subsequent
history has been ever based upon the same principle. There has been a
constant separation of the different functions of life among groups of
cells, and as the history went on this division of labor among the
different parts became greater and greater. Group after group of cells
were set apart for one special duty after another, and the result was a
larger and ever more complicated mass of cells, with a greater and
greater differentiation among them. In this building of the machine
there was no time when the machine was not active. At all points the
machine was alive and functional, but each step made the total function
of the machine a little more accurately performed, and hence raised
somewhat the totality of life powers. This parcelling out of the
different duties of life to groups of cells continued age after age,
each step being a little advance over the last, until the result has
been the living machine as we know it in its highest form, with its
numerous organs, all interrelated in such a way as to form a
harmoniously acting whole.

[Illustration: FIG. 46. A later step in machine building in which the
outer cells have acquired different form and function from the inner
cells: _ec_, the outer cells, whose duties are protective; _en_, the
inner cells engaged in digesting food.]

But a second principle in this growth of the machine was needed to
produce the variety which is found in na