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 is what we sometimes call teleological, the latter scientific; the former was the attitude of the study of animals and plants before the middle of this century, the latter the spirit which actuates modern biology.

==The Mechanical Nature of Living Organisms.==—­This new attitude forced many new problems to the front.  Foremost among them and fundamental to them all were the questions as to the mechanical nature of living organisms.  The law of the correlation of force told that the various forms of energy which appear around us—­light, heat, electricity, etc.—­are all parts of one common store of energy and convertible into each other.  The question whether vital energy is in like manner correlated with other forms of energy was now extremely significant.  Living forces had been considered as standing apart from the rest of nature. Vital force, or vitality, had been thought of as something distinct in itself;