That specific substances may have an influence on the growth of certain parts cannot be denied, but it appears that in general they play a very secondary rôle as compared with other factors that determine the form of the organism or the development of a part. Vöchting’s beautiful experiments (’86) on tuberous plants show that the presence of an excessive amount of food substances in the plant, brought about by the artificial removal of the natural storehouses for such material, may act on certain parts, such as the axial buds, or on the stem, and cause them to produce structures that they do not produce under ordinary circumstances. The axial buds become swollen and produce tuber-like bodies above ground, especially if the parts are enclosed so as to be in the dark, since the light retards the growth of tubers of all sorts. But it should not be overlooked that these buds and stems are structurally the same things as the tuberiferous stolons that have been removed, and hence the excess of material is stored up in them in the same way as it is under normal circumstances in the underground stems or stolons. The reaction is one normal to the plant, although it usually takes place in a different part. The preceding hypotheses that have been advanced to account for the phenomena of regeneration, draw attention to some of the most fundamental problems of regeneration and, even in those cases in which the hypotheses have not given a satisfactory solution of the problems, some of them have served the good purpose, both of directing attention to important questions and of leading biologists to make experiments to test the new points of view. We should not underrate their value, even if they have sometimes failed to give a solution of problems, for they have been useful if only in eliminating certain possibilities, and this simplifies all future work. So long as an hypothesis is of a sort that it is within the range of observational and experimental test, it may be of service, even if it prove erroneous; for our advance through the tangled thread of phenomena is not only assisted by advances in the right direction, but all possibilities must be tested before we can be certain that we have discovered the whole truth. The value of a scientific hypothesis depends, it seems to me, first, on the possibility of testing it by direct observation, or by experiment; second, on whether it leads to advance; and, lastly, on its elimination of certain possibilities.