THEORIES OF REGENERATION 2?$ 



tern. The new rearrangement appears to take place on the founda- 

 tions of the old system. 



It may appear that this idea of a system of tensions is too vague, 

 that it fails to point out how the reorganization takes place, and that 

 it gives not much more than the facts do themselves. There is a 

 certain amount of truth in these objections which I fully appreciate, 

 but something further can be said on these points. The view is 

 vague in so far as we cannot picture to ourselves in a mechanical 

 way just how such a system could bring about the suppression of 

 growth in one region and allow the maximum amount in another 

 region. But this is asking too much, since the hypothesis can only 

 claim, at present, to furnish a means by which we can at least 

 imagine what sort of a process is involved, and cannot give the 

 details of the process itself. It can be shown experimentally that if 

 the phenomenon is one of tension certain results should follow that 

 are observed to take place, as when by keeping the shorter half of 

 the planarian from reuniting to the larger half, or by breaking the 

 union if it has been formed, a head develops also at the posterior 

 cross-cut. In the second place, although we cannot understand how 

 the rearrangement of the tensions in a piece takes place, yet from 

 a causal point of view we can see how a change in one region of a 

 labile system may produce, by means of a change of tension, a com- 

 plete rearrangement of the parts throughout. It can even be 

 claimed for the tension hypothesis that it at least becomes easier for 

 us to see how such a change could take place, because it represents 

 the organization as the expression of a system under tension, and 

 hence, if the material is sufficiently flexible, a readjustment will proba- 

 bly take place when the system is changed in any region. It enables 

 us to see how the organization of the egg may be divided by every cell 

 division, and yet after the reunion of the cells the original equilibrium 

 be established. We may perhaps claim, therefore, that in these re- 

 spects the hypothesis does give us something more than do the 

 facts ; and, inasmuch as it brings a large number of phenomena under 

 a common point of view, the idea may be worth further consideration. 



In conclusion, I may add that the hypothesis is, I hope, also a legit- 

 imate one, in the sense that being within reach of an experimental proof 

 or disproof, it may serve at least as a working hypothesis. Per- 

 haps more fundamental than the idea that a system of tensions 

 exists throughout the organization is the conception that the organi- 

 zation is itself a system of interrelated parts, and not a homogeneous 

 substance or a mass composed of a large number of repeated parts, 

 or rather, despite the presence of smaller, repeated units, the organi- 

 zation is not the result of their interaction, but of their regular 

 arrangement as parts of a whole structure. If, then, this inter- 



