286 REGENERATION 



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 devel- 

 opment of a structure, the gall, of a definite form. The experiments 

 of Herbst on the effect of lithium salts in sea water on the develop- 

 ment 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." 1 The effect of inter- 

 nal 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 account such properties of bodies as those 

 which the chemist calls the affinities of atoms and molecules, or the 

 symmetrical deposition of material on the surface of a crystal from a 

 supersaturated solution, etc. These phenomena are usually looked 

 upon as " given," that is, beyond the hope of possible examination. 

 Until these questions are more fully understood scientists are, I 

 think, justified in showing a certain amount of self-restraint in regard 

 to the solution of such problems. Du Bois-Reymond has summed 

 up this point of view in the dictum, " Ignorabimus," which is inter- 

 preted to mean, not only that we are ignorant at present on certain 

 questions, but that we know we must remain ignorant. The forma- 

 tive changes in the organism appear to belong to this category of ques- 

 tions. This confession of ignorance need not mean that we cannot 

 hope to discover the conditions under which the phenomena take place, 

 so that we can predict with certainty what the results will be, but 



1 Not that Driesch supposes this would be the case. 



