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 living matter of 
forces, or energies, or factors that are not exhibited by non-living 
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 little 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 familiar 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. 
