THEORIES OF REGENERATION 269 



determine the form of the organism or the development of a part. 

 Vochting'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 pro- 

 duce 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 direct- 

 ing 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 experi- 

 ment ; second, on whether it leads to advance ; and, lastly, on its 

 elimination of certain possibilities. 



The experiments described in Chapters II, III, IV, have shown that 

 there are many resemblances between the phenomena of growth and 

 of regeneration. It has been pointed out that when it could be shown 

 that certain external agents have a determining influence upon growth, 

 these same agents have a similar effect upon regeneration. This 

 also holds apparently for internal factors, although it is much more 

 difficult to demonstrate that this is true. The presence of an abun- 

 dance of food material in the tissues hastens regeneration in the same 

 way that growth is more rapid in a well-fed organism. Food may, 

 however, be looked upon rather as an external factor than as an 



