=New Aspects of Biology==.--These three conceptions seized hold of the
scientific world at periods not very distant from each other, and their
influence upon the study of living nature was immediate and
extraordinary. Living things now came to be looked upon not simply as
objects to be catalogued, but as objects which had a history, and a
history which was of interest not merely in itself, but as a part of a
general plan. They were no longer studied as stationary, but as moving
phases of nature. Animals were no longer looked upon simply as beings
now existing, but as the results of the action of past forces and as the
foundation of a different series of beings in the future. The present
existing animals and plants came to be regarded simply as a step in the
long history of the universe. It appeared at once that the study of the
present forms of life would offer us a means of interpreting the past
and perhaps predicting the future.

In a short time the entire attitude which the student assumed toward
living phenomena had changed. Biological science assumed new guises and
adopted new methods. Even the problems which it tried to solve were
radically changed. Hitherto the attempt had been made to find instances
of _purpose_ in nature. The marvellous adaptations of living beings to
their conditions had long been felt, and the study of the purposes of
these adaptations had inspired many a magnificent conception. But now
the scientist lost sight of the purpose in hunting for the _cause._
Natural law is blind and can have no purpose. To the scientist, filled
with the thought of the reign of law, purpose could not exist in
nature. Only cause and effect appeal to him. The present phenomena are
the result of forces acting in the past, and the scientist's search
should be not for the purpose of an adaptation, but for the action of
the forces which produced it. To discover the forces and laws which led
to the development of the present forms of animals and plants, to
explain the method by which these forces of nature have acted to bring
about present results, these became the objects of scientific research.
It no longer had any meaning to find that a special organ was adapted to
its conditions; but it was necessary to find out how it became adapted.
The difference in the attitude of these two points of view is
world-wide. The former fixes the attention upon the end, the latter upon
the means by which the end was attained; the former