GENERAL CONSIDERATIONS AND CONCLUSIONS 287 



the meaning of the change itself may remain forever obscure, at 

 least from our present conception of physico-chemical principles. 

 Shall we, therefore, call ourselves vitalists, because we find certain 

 phenomena that we cannot hope to explain as the result of physical 

 principles, or for which we must invent an unknown principle ? Or 

 can we succeed in demonstrating a different kind of principle in liv- 

 ing things ? If we could, we might be justified in calling ourselves by 

 the name of vitalists. But who has made such a discovery ? Does 

 the well-known phenomenon of proportionate development give a 

 demonstration of the unknown principle? Would one be justified 

 in claiming a different principle that is not a physico-causal one, 

 because the nerve impulse is different from any known physical phe- 

 nomenon ? The preceding pages have made clear, I hope, that, for 

 my own part, I see no grounds for accepting a vitalistic principle 

 that is not a physico-causal one, but perhaps a different one from 

 any known at present to the physicist or chemist. 



In order to make clear in what way certain terms have been used 

 in the preceding chapters, it may not be out of place to indicate how 

 it is intended that they should be employed. The word " cause " has 

 been used in the sense in which the physicist uses the term. A "stim- 

 ulus" is the chain of effects of a cause acting on a living body. In cer- 

 tain cases the cause itself may be spoken of as the stimulus, but 

 only when its specific action on a living body is implied. A "factor" 

 is a more general term and is usually one or more of a number of 

 causes that produce a result. It may prove convenient to use this term 

 where a change in form is produced. Thus the size of a piece is one 

 of the factors that determines the result ; the part of the body from 

 which the piece is taken may also be a factor, -or rather the kind of 

 material contained in the piece. These examples will suffice to show 

 that the word is used for an observed connection of a very general 

 sort, especially for those cases in which we have not analyzed the 

 factor into its components. The term is especially useful for cases 

 in which the change in form is the outcome of the innate properties 

 of the organization. The term may be used so that it need not preju- 

 dice the result, either in favor of a physico-causal or a vitalistic- 

 causal point of view. It may be convenient to use it as an indifferent 

 term in these respects. The word "force " I have attempted to avoid 

 as far as possible, except in such current expressions as "the force of 

 gravity," etc., for, apart from the loose way in which the word is used 

 even by physicists, we know so little about the forces in the organ- 

 ism that it is best, I think, to use the word as sparingly as possible, 

 and only where a known physical force can be shown to produce an 

 effect. 



