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 complete 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 probably 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 respects 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.