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The problem of vitalism need be very briefly examined. Vitalism, if it means anything in biology, interprets life in terms of forces or agencies or processes that are not found in inorganic nature. According to this definition, Aristotle was a vitalist when he conceived the development of the germ to be guided by the entelechies that determined specific and individual form in organisms. Wolff was a vitalist when he accounted for the differentiation of a homogeneous germ by the aid of a vis essentialis. Vital forces have long since lost their grip. They began to weaken when Wöhler, in 1828, produced in the laboratory the compound urea, till then supposed to be formed only in the bodies of organisms. They broke into full retreat under the fire of calorimetric researches of the last century which demonstrated that oxidation was oxidation, whether it took place within or without the body, and that vital heat was as surely due to chemical reaction as the heat generated by the reaction between sulphuric acid and zinc.

So Wolff's vitalism is dead. The Aristotelian vitalism, however, has a representative at the present day in the neo-vitalism of Driesch. The Aristotelian entelechy has been revamped and applied to the unexplained residuum that has escaped Driesch's experimental analysis. It is interesting that Driesch was a metaphysician first, an experimental biologist second; and that after about fifteen years of unusual activity in this second rôle, he returned to his first love. In these fifteen years he developed what he has called three proofs of vitalism. But he has not succeeded in persuading many biologists to accept his criteria of demonstration. It is difficult to take seriously his conception of entelechy, a non-substantial, non-energetic principle which yet is competent to control the developmental energies of the organism. It is but another final cause, an ultimate term in the analysis of the activities of organisms. And it has weakened Driesch's interest in biological research just as the formulation of final explanations has led to stagnation wherever we have met them along the line of biological inquiry.

In contrast with Driesch, there is a large and eager group of experimental biologists who unite in deprecating his interest in entelechies and, undaunted by its enormous complexity, in investigating the organic mechanism in the hope of reducing more of it than he was able, to terms of physics and chemistry. How far they may go is not, from the standpoint of modern biology, a pertinent question. How they may keep moving is more to the point. To this end the Drieschian entelechy offers not the slightest suggestion of encouragement.

Three of the four problems to which attention was invited at the beginning of this paper have now been considered. If I have succeeded