no need and no presence of claws or
carapace; where sting or venom, no need and no possession of odor,
claws, shell, extraordinary strength, or sagacity. Where the struggle is
most bitter, there exist the most complex and most numerous contrivances
for living.

Throughout its whole course the process of evolution, where it is
visible in the struggle of organisms, has been marked by a progressive
victory of brain over brawn. And this, in turn, may be regarded as but a
manifestation of the process of survival by _lability_ rather than by
_stability_. Everywhere the organism that exhibits the qualities of
quick response, of extreme sensibility to stimuli, of capacity to
change, is the individual that survives, "conquers," "advances." The
quality most useful in nature, from the point of view of the domination
of a wider environment, is the quality of _changeableness_,
_plasticity_, _mobility_, or _versatility_. Man's particular means of
adaptation to his environment is this quality of versatility. By means
of this quality expressed through the manifold reactions of his highly
organized central nervous system, man has been able to dominate the
beasts and to maintain himself in an environment many times more
extensive than theirs. Like the defensive mechanisms of shells, poisons,
and odors, man's particular defensive mechanism--his versatility of
nervous response (mind)--was acquired automatically as a result of a
particular combination of circumstances in his environment.

In the Tertiary era--some twenty millions of years ago--the earth,
basking in the warmth of a tropical climate, had produced a luxuriant
vegetation and a swarming progeny of gigantic small-brained animals for
which the exuberant vegetation provided abundant and easily acquired
sustenance. They were a breed of huge, clumsy, and grotesque monsters,
vast in bulk and strength, but of little intelligence, that wandered
heavily on the land and gorged lazily on the abundant food at hand. With
the advance of the carnivora, the primitive forerunners of our tigers,
wolves, hyenas, and foxes, came a period of stress, comparable to a
seven years of famine following a seven years of plenty, which subjected
the stolid herbivorous monsters to a severe selective struggle.

Before the active onslaught of lighter, lither, more intelligent foes,
the clumsy, inelastic types succumbed, those only surviving which,
through the fortunate possession of more varied reactions, we