changing environment, changing according to law, called for
ceaseless readjustment, and the adaptation to environment was held to be
the stimulus to all activity in the natural world.

The later development of biology, and the doctrine of the evolution of
species, gradually extended this conception of nature to include man
himself.

What he had regarded as his distinctive characteristics were held to be
but the product of natural factors, and his life was regarded, too, as
under the domain of rigid, inviolable law. There was no room for, and no
need of, the conception of free, originative thought. Thought was
simply an answer to the demand that the sense world was making, entirely
dependent upon the external stimulus, just as any other activity was
entirely dependent on an external stimulus. So thought came to be
regarded as resulting from mere sense impression, which latter
corresponded to the external stimulus. It is obvious that the idea of
the freedom of the human soul, and of human personality as previously
understood, had to go. Man was simply the result of the interaction of
numerous causes--and like the rest of nature, involved no independent
spiritual element. Everything that was previously regarded as spiritual
was interpreted as a mere adjunct to, or a shadow of, the sense world.
Such a conception accounted for the whole of nature and of man, and so
became an explanation of the universe, a philosophy.

In such a theory self-preservation becomes the aim of life, the struggle
for existence the driving-power, and adaptation to environment the means
to the desired end. Hence it comes about that only one standard of value
remains, that of usefulness, for that alone can be regarded as valuable
which proves to be useful towards the preservation and enjoyment of the
natural life. The ideas of the good, the beautiful, and the true, lose
the glory of their original meaning, and become comparatively barren
conceptions. Hence at a stroke the spiritual world is wiped away, the
soul of man is degraded from its high position, the great truths of
religion are cast aside as mere illusions.

The naturalistic explanation possesses the apparent advantage of being a
very simple one, and hence attracts the human mind with great force in
the early stages of mental culture. All the difficulties of the
conception of a higher world are absent, for the naturalistic position
does not admit of its existence. It gives, too, some purpo