THEORETIC CONSIDERATIONS 371 



of organisms by chemical changes which it causes within 

 them, is as yet founded almost entirely on such hypo- 

 thetical assumptions — assumptions which are problems, 

 not solutions. 



(2) Loeb's explanations of the origin of adaptive reac- 

 tions to light is found in the following quotation (1906, 

 p. 160): " The fact that cases of tropism occur even where 

 they are of no use, shows how the play of the blind forces 

 of nature can result in purposeful mechanisms. There is 

 only one way by which such purposeful mechanisms can 

 originate in nature; namely, by the existence in excess of 

 the elements that must meet in order to bring them about. 

 In green plants and in some animals the positive heliotrop- 

 ism is useful; yet there exists probably an endless number 

 of heliotropic animals for which their heliotropism is about 

 as useless as is galvanotropism. The prerequisites for 

 heliotropism are a symmetrical body form, which seems 

 to be present in almost all organisms — although some 

 asymmetries exist — and the presence of photosensitive 

 substances, which is not quite so common, but certainly 

 not infrequent. Some of the regular substances found 

 in protoplasm seem to turn readily into a photosensitive 

 form. As the two conditions mentioned above are quite 

 common, the laws of probability make it necessary that 

 in a certain number of cases both conditions will be fulfilled, 

 and then we may expect heliotropic actions. If it now 

 occurs that in an organism the turning to the light helps 

 it to find its food, as is the case with certain caterpillars, 

 e.g., Porthesia chrysorrhoea, or the stems of green plants 

 whose starch is manufactured by light, we have a ' purpose- 

 ful mechanism.' Again, according to the laws of probabil- 

 ity, the number of animals in which the three groups of 

 conditions meet is much smaller than where only two meet. 

 The tropisms thus furnish an insight into the origin of 

 purposeful reactions by the blind forces of nature." The 

 difficulty with this hypothesis is that it does not fit the 

 facts. It rests primarily upon the assumption that there 



