If we cannot accept Driesch’s demonstration of vitalism, from what point of view can we deal with the phenomenon of the production of a typical form from each kind of living material? Can we find a physico-chemical explanation of the phenomenon? Enough has been said to show that this property is one of the fundamental characteristics of living things and is, in all probability, a phenomenon which we certainly cannot at present hope to explain. Yet the question raised by Driesch is, at bottom, not so much whether we can give a physico-chemical explanation, but whether the phenomenon belongs to an entirely different class of phenomena from that considered by the physicist and by the chemist. Let us examine the results and see if we are really forced to conclude that there is no other physico-causal point of view possible. In many cases in which a response to an external stimulus takes place, we must assume a physico-causal connection between the stimulus and the effect. The action of poisons, for instance, is an example of this kind, and, in some cases, as in the formation of the galls of plants, the stimulus of a foreign body may lead to the development of a structure, the gall, of a definite form. The experiments of Herbst on the effect of lithium salts in sea water on the development of the sea-urchin embryo lead to a similar conclusion. The changes in form that result from other external agents, such as light, gravity, contact, etc., can be best understood from a physico-causal point of view, and it seems improbable at least that their action within the organ is transformed into a vitalistic causal action through Driesch’s principle of an “action at a distance.”[133] The effect of internal factors on the change of form is, however, much more difficult to deal with, since we know so little at present about these factors. Here we find amongst other phenomena that of the proportionate formation of a whole organ from a part of an old one, or of an egg. We find it difficult, if not impossible, to attribute this directly to external causes, yet, as I have tried to show, the first steps through which this takes place can be referred to physico-causal principles. These are the separation of the piece from the whole; the change of the unsymmetrical piece into a symmetrical one, brought about, in part at least, by contractile phenomena in the piece, aided, no doubt, in some cases by surface tension, etc. These changes give the basis for the development of a new organization along the lines of structure that are already present in the piece. We find here the beginning of a physico-causal change, and, so far as I can see, we have no reason to suppose that at one stage in the process this passes over into the vitalistic-causal principle. It should, I think, be pointed out in this connection that even in the physical sciences it would not be difficult to establish a vitalistic principle, or whatever else it might be called, if we chose to take into