and the mistake in supposing that the American
forms grew smaller than their ancestors in the Old World, certainly
smack of the principle of isolation and segregation, and this is
Buffon's most important contribution to the theory of descent.

"It is probable, then, that all the animals of the New World are
derived from congeners in the Old, without any deviation from the
ordinary course of nature. We may believe that, having become
separated in the lapse of ages by vast oceans and countries which
they could not traverse, they have gradually been affected by, and
derived impressions from, a climate which has itself been modified
so as to become a new one through the operations of those same
causes which dissociated the individuals of the Old and the New
World from one another; thus in the course of time they have grown
smaller and changed their characters. This, however, should not
prevent our classifying them as different species now, for the
difference is no less real though it dates from the creation.
_Nature, I maintain, is in a state of continual flux and movement.
It is enough for man if he can grasp her as she is in his own time,
and throw but a glance or two upon the past and future, so as to try
and perceive what she may have been in former times and what one day
she may attain to._"[137]

Buffon thus suggests the principle of the struggle for existence to
prevent overcrowding, resulting in the maintenance of the balance of
nature:

"It may be said that the movement of Nature turns upon two immovable
pivots--one, the illimitable fecundity which she has given to all
species; the other, the innumerable difficulties which reduce the
results of that fecundity, and leave throughout time nearly the same
quantity of individuals in every species; ... destruction and
sterility follow closely upon excessive fecundity, and,
independently of the contagion which follows inevitably upon
overcrowding, each species has its own special sources of death and
destruction, which are of themselves sufficient to compensate for
excess in any past generation."[138]

He also adds, "The species the least perfect, the most delicate, the
most unwieldy, the least active, the most unarmed, etc., have already
disappeared or will disappear."[139]

On one occasion, in writing on the dog, he anticipates Erasmus Darwin
and Lamarck in ascribing to the direct cause of modification the inne