form the essential arguments that he had already put forward in more extended papers on the development of fractional parts of the egg and on the general problem of the localization of morphogenic processes, Driesch maintained that these processes have no true analogue in the inorganic world, are insoluble by any purely mechanical or physico-chemical hypothesis, and hence form a problem sui generis. The most characteristic operations of the living organism, more especially those concerned in the processes of regeneration, regulation and the like, fail of adequate interpretation on the so-called 'machine-theory' of life, and must be regarded from a vitalistic as opposed to a mechanistic standpoint. His conclusions, which were stated with great lucidity and force, met with strong opposition in the animated controversy that followed, in which a number of eminent embryologists participated. Some of these speakers wholly denied the validity of Driesch's reasoning and endeavored to show that true analogues to regulative phenomena occur in purely physical processes. Others, notably Professor Roux, took more cautious ground, maintaining that despite our present inability to explain or even to conceive the nature of some of the most striking and characteristic phenomena of development, we are by no means justified in taking refuge behind such a word as 'vitalism,' which carries with it so many misleading connotations from the earlier period when it was employed in connection with the. exploded hypothesis of a specific 'vital force.'
The masterly and scholarly address of Professor Bütschli, delivered before the general session, contained not only a specific examination of the main facts in which Driesch's position rests, but also a critical study of more abstract conceptions, such as those embodied in the words 'mechanism,' 'causality' and the like, which are inevitably involved in the discussion of the subject. This address, which has been published in pamphlet form by Engelmann of Leipzig under the title Mechanism and Vitalism is worthy of attentive study, not only by students of zoology, but also by all who are interested in the more general aspects of scientific progress. Recognizing the difficulties that the mechanistic interpretation of organic nature has to encounter, Bütschli nevertheless expresses the judgment that, in the broad sense of the phrase, it is the only one under which scientific investigation is possible, and that it is, to say the least, wholly premature to speak of 'proofs of vitalism.' "The phenomena involved in the localization process seem to me not to differ fundamentally in kind from those occurring in the inorganic world." The acceptance of vitalistic hypothesis constitutes a backward step in scientific method. "Both the old and the new vitalism have done no more than to emphasize the unsolved riddles that confront us and to throw doubt on the possibility of their explanation on a mechanistic basis. The assumption of vitalistic processes involves the admission that they are ultimate phenomena, in themselves inexplicable, that we are not able to subsume under general laws. Hence we must take the ground that in vital phenomena we can comprehend only that which may receive a physicochemical explanation." How far the mechanistic hypothesis will succeed in the explanation of vital phenomena, only the future will show. 'By their fruits ye shall know them.'
In considering the possibility of a mechanistic explanation of the purposive or teleological aspect of living organisms Bütschli recognizes Darwin's theory of natural selection as the sole fruitful attempt in this direction. In view of the difficulties that have been urged against that theory, and especially the drastic criticism it has received at the hands of some German writers, it is interesting to find that