Experiments in Grafting 297 



explain the results as there are different species of animals hav- 

 ing this power. In other words, the use of the term "formative 

 force" is only a restatement of the problem, not a causal ex- 

 planation of it. Perhaps the main objection to this, and to 

 similar terms, is that they imply the existence in hving matter of 

 forces, or energies, or factors that are not exhibited by nonliving 

 things. In so far as there is postulated only a different kind of 

 physical action from any so far described by physicists, little 

 objection can be raised ; but until the nature of this new force can 

 be demonstrated, very httle if anything is gained by assuming 

 its existence. If, on the other hand, the postulated principle 

 is supposed to be different in character from all other physical 

 events, then the matter becomes more serious ; for the assump- 

 tion is either metaphysical, and therefore outside of the proper 

 field of science, or if not metaphysical, the assumption attempts 

 to account for known events by a principle entirely unknown. 

 In the latter case nothing is gained, and since the nature of the 

 question itself is prejudged, harm may be done. In the preceding 

 pages, in attempting to account for the changes in hydra, etc., 

 I have assumed that the formative changes are the outcome of 

 a relation of tension in the parts. From this point of view the 

 condition of tension is the stimulus to which the material basis 

 of the organism responds. 



The postulated factor is a physical one, and the response of the 

 cells that determines the result is supposed to be, in most cases, 

 the famihar response of contraction shown by all animals. In 

 this respect the nature of the process is assumed to be the same as 

 that seen in other phenomena involving contact and response by 

 contraction. There is nothing in the nature of this reaction that 

 seems to preclude a purely physical process. In addition, how- 

 ever, it is necessary to assume that the differentiation of the cells 

 also takes place as a result of the mutual pressure of the parts 

 *on each other. This assumption is more arbitrary and more 

 difficult to bring into accord with our present knowledge ; yet the 

 facts seem to demand, I think, some such view, although at pres- 

 ent it can only be offered as a provisional or working hypothesis. 



