that growth is
substantially equivalent to the absorbed nutriment minus the nutriment
used up in action. This, however, does not account for the fact that in
every domestic animal the increments of growth bear continually
decreasing ratios to the mass, and finally come to an end. Nevertheless,
it is demonstrable that the excess of absorbed over expended nutriment
must decrease as the size increases. Since in similar bodies the areas
vary as the squares of the dimensions and the masses vary as the cubes,
it follows that, however great the excess of assimilation over waste may
be during the early life of an active organism, there must be reached,
if the organism lives long enough, a point at which the surplus
assimilation is brought to nothing--a point at which expenditure
balances nutrition, a state of moving equilibrium. Obviously, this
antagonism between assimilation and expenditure must be a leading cause
of the contrast in size between allied organisms that are in many
respects similarly conditioned.

_Development, or Increase of Structure_

In each of the organic sub-kingdoms the change from an incoherent,
indefinite homogeneity to a coherent definite heterogeneity is
illustrated in a quadruple way. The originally-like units or cells
become unlike, in various ways, and in ways more numerously marked as
the development goes on. The several tissues which these several classes
or cells form by aggregation, grow little by little distinct from each
other; and little by little become structurally complex. In the shoot as
in the limb, the external form, originally very simple and having much
in common with countless simple forms, organic and inorganic, gradually
acquires an increasing complexity, and an increasing unlikeness to other
forms, and meanwhile, the remaining parts of the organism, having been
developed severally, assuming structures diverging from each other and
from that of this particular shoot or limb, there has arisen a greater
heterogeneity in the organism as a whole.

The most remarkable induction of von Baer comes next in order. It is
that in its earliest stage every organism has the greatest number of
characters in common with all other organisms in their earliest stages;
that at each subsequent stage traits are acquired which successively
distinguish the developing embryo from groups of embryos that it
previously resembled--thus step by step diminishing the group of embryos
which it still resembles