made along some one of the various lines of research, and to the milestones which mark the epochs of advance along the way which science has traveled. Moreover, we may profitably sum up at such times the work done in particular directions, and encourage ourselves with prospective and retrospective glances. In these summings up, however, a difficulty arises. The range of modern scientific thought includes an immense area. The field of knowledge is already so vast that, seen from the vertical distance necessary to make a wide survey, that small portion of it which is familiar to any one individual is scarcely visible. In consequence, to use a mechanical figure, the solid contents of a man's acquirements being given, the depth thereof is inversely as the area covered. He, therefore, who undertakes to speak even for one single department of science distributes his stock of knowledge over so broad a surface that in places it must become dangerously thin. It is, therefore, with a very keen sense of the temerity involved in the undertaking that I ask your attention, during the hour allotted me, to some points which appear to me to have been recently gained in the discussion of the question of life.
My friend and predecessor, Professor Marsh, opened his excellent address at Saratoga with the question, "What is Life?" In a somewhat different sense I too ask the same question. But I fear it is only to echo his reply, "The answer is not yet." The result, however, can not long be doubtful. "A thousand earnest seekers after truth seem to be slowly approaching a solution." And, though the ignis fatuus of life still dances over the bogs of our misty knowledge, yet its true character can not finally elude our investigation. The progress already made has hemmed it in on every side; and the province within which exclusively vital acts are now performed narrows with each year of scientific research.
What now are we to understand by the word "Life" in this discussion? A noteworthy parallel is disclosed in the progress of human knowledge between the ideas of life and of force. Both conceptions have advanced, though not with equal rapidity, from a stage of complete separability from matter to one of complete inseparability. Life is now universally regarded as a phenomenon of matter, and hence, of course, as having no separate existence. But there still exists a certain vagueness in the meaning of the term "life." Two distinct senses of this word are in use; the one metaphysical, the other physiological. The former, synonymous with mind and soul, at least in the higher animals, has been evolved from human consciousness; the latter has arisen from a more or less careful investigation of the phenomena of living beings. It need scarcely be said that it is in the sense last mentioned that the word "life" is used in science. The conception represents simply the sum of the phenomena exhibited by a living being.
Moreover, the progress which has been made in the solution of the life-question has been gained chiefly by investigation of special func-
