; and that thus the class of similar forms is
finally narrowed to the species of which it is a member. For example,
the human germ, primarily similar to all others, first differentiates
from vegetal germs, then from invertebrate germs, and subsequently
assumes the mammalian, placental unguiculate, and lastly the human
characters.

The development of an individual organism is at the same time a
differentiation of its parts from each other and a differentiation of
the consolidated whole from the environment; and in the last as in the
first respect there is a general analogy between the progression of an
individual organism and the progression of the lowest orders of
organisms to the highest orders.

_The Laws of Multiplication_

Every living aggregate being one of which the inner actions are adjusted
to balance outer actions, it follows that the maintenance of its moving
equilibrium depends on its exposure to the right amounts of these
actions. Its moving equilibrium may be overturned if one of these
actions is either too great or too small in amount: either by excess or
defect of some inorganic or organic agency in its environment.

Our inquiry resolves itself into this:--in races that continue to exist
what laws of numerical variation result from these variable conflicting
forces?

The forces preservative of a race are two--ability in each member of the
race to preserve itself, and ability to produce other members. These
must vary inversely--one must decrease as the other increases. We have
to ask in what way this adjustment comes about as a result of evolution.

Including under individuation all those processes completing and
maintaining individual life, and under genesis all those aiding the
formation and perfecting of new individuals, the two are necessarily
antagonistic. Every higher degree of individual evolution is followed by
a lower degree of race multiplication, and _vice versa_. Progress in
bulk, complexity or activity involves retrogress in fertility; and
progress in fertility involves retrogress in bulk, complexity, or
activity. The same quantity of matter may be divided into many small
wholes or few large wholes; but number negatives largeness, and
largeness negatives number.

It is a general physiological truth that while the building-up of the
individual is going on rapidly, the reproductive organs remain
imperfectly developed and inactive; and that the commencement of
reproduction at once indi