l6o DYNAMICS OF LIVING MATTER 



the principle of natural selection, which has certainly been a factor in 

 the elimination of forms, although it played no rdle in producing any 

 qualities or irritabilities. The fact that cases of tropism occur even 

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

 ture 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 heliotropism 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 organ- 

 isms — although some asymmetries exist — and the presence of photo-^ 

 sensitive 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 men- 

 tioned above are quite common, the laws of probability make it neces- 

 sary 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 chrysorrhcea, or the stems of 

 green plants whose starch is manufactured by hght, we have a "purpose- 

 ful mechanism." Again, according to the laws of probability, 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. 



