s and plants simply as so many museum specimens to be arranged on
shelves with appropriate names. The modern biologist is studying these
same objects as intensely active beings and as parts of an ever-changing
history. To the student of natural history fifty years ago, animals and
plants were objects to be _classified_; to the biologist of to-day, they
are objects to be _explained_.

To understand this new attitude, a brief review of the history of the
fundamental features of philosophical thought will be necessary. When,
long ago, man began to think upon the phenomena of nature, he was able
to understand almost nothing. In his inability to comprehend the
activities going on around him he came to regard the forces of nature as
manifestations of some supernatural beings. This was eminently natural.
He had a direct consciousness of his own power to act, and it was
natural for him to assume that the activities going on around him were
caused by similar powers on the part of some being like himself, only
superior to him. Thus he came to fill the unseen universe with gods
controlling the forces of nature. The wind was the breath of one god,
and the lightning a bolt thrown from the hands of another.

With advancing thought the ideas of polytheism later gave place to the
nobler conception of monotheism. But for a long time yet the same ideas
of the supernatural, as related to the natural, retained their place in
man's philosophy. Those phenomena which he thought he could understand
were looked upon as natural, while those which he could not understand
were looked upon as supernatural, and as produced by the direct personal
activity of some divine agency. As the centuries passed, and man's power
of observation became keener and his thinking more logical, many of the
hitherto mysterious phenomena became intelligible and subject to simple
explanations. As fast as this occurred these phenomena were
unconsciously taken from the realm of the supernatural and placed among
natural phenomena which could be explained by natural laws. Among the
first mysteries to be thus comprehended by natural law were those of
astronomy. The complicated and yet harmonious motions of the heavenly
bodies had hitherto been inexplicable. To explain them many a sublime
conception of almighty power had arisen, and the study of the heavenly
bodies ever gave rise to the highest thoughts of Deity. But Newton's law
of gravitation reduced the whole to the greates