328 CHARLES S. MYEES : protoplasm may be made to grow and to continue its usual functions amidst conditions, which would have killed it had it been ruthlessly plunged from the normal into this toxic medium. Evidently the chemical constitution of the proto- plasmic molecule is thereby changed ; the living substance may be said to have been educated to act in a strange environment. The persistent application of such abnormal stresses from without might conceivably bring about a per- manent change in the chemical composition of protoplasm, thereby giving rise to different individuals or even to different species. It is, however, impossible to demonstrate the permanent modification of the protoplasm of any metazoon by a new constant stress from without, and hence an expres- sion of the phenomena of heredity, evolution and development in such simple physical language falls to the ground. On the other hand, development has too long been ascribed to the effects of a closed internal system, independent of all external influence ; while qualities, admittedly complex, have been retained in an invisible world of ids and determinants. Only by attributing the various changes of the embryonic and adult life to the interaction of external and internal conditions, will further experiments be prompted to settle such disputed questions as the influence of external conditions on the determination of sex, function and monstrosities. This brief mention of Weismaun's theory here in con- nexion with the difficulties of a mechanical description of vital phenomena would have little to recommend itself, were it not for the great moral the theory carries with it. The principle of the non-transmission of acquired characters, which, however incomplete, still remains unassailed by direct experiment, was directly deduced by Weismaun from his inductively formed theory of heredity. Indeed so many are the discoveries, so numerous the researches which have been called forth by its aid that, even when rejected, this theory is destined to an immortal place in the annals of Natural Science. Concerning the nature of life, as concerning the nature of heredity, it is the imperative duty of the biologist to brush from his eyes the veil of hopeless pessimism, to look carefully and impartially on all sides of him, to frame the best theory which knowledge thus acquired permits him, and to test the truth of his theory by such experiments as it suggests. The conception of life as a special form of activity, has been discussed in the light of two alternatives. Either this activity is the expression of mechanical forces which, to use Virchow's words, "act under most extraordinary and varied
