60 The Mechanistic Conception of Life 



result of secondary and often minor complications; the latter 

 investigators have from this standpoint made further fruitful 

 discoveries. The role of the other group of investigators in 

 this case has consisted, primarily, in an attempt to minimize 

 the importance of Mendel's laws and thus to retard the progress 

 of science. 



The case is similar in the realm of tropisms. Tropisms 

 and tropism-like reactions are elements which pave the way 

 for a rationalistic conception of the psychological reactions of 

 animals and I believe, therefore, that it is in the interest of the 

 progress of science to develop further the theory of animal 

 tropisms. The fact that in an electric current the same animal 

 often moves differently from what it does under the influence 

 of light finds its explanation for the observer conversant with 

 physical chemistry in the fact that the electric current causes 

 changes in the concentration of ions within, as well as upon the 

 surface, while the chemical action of light is essentially limited 

 to the surface. Certain writers, however, leave this difference 

 in the action of the two agents out of consideration and make 

 use of the difference in the behavior of certain organisms in 

 response to light and to the electric current, to assert that it 

 is not permissible to speak of tropisms as being governed 

 by general laws; in other words, they say that tropisms are 

 without significance. Animals in general are symmetrically 

 built and the motor elements of the right and left sides of the 

 body usually act symmetrically. Consequently the heliotropic 

 orientation, for instance, comes about as we have already 

 described. There are animals, however, which move sideways, 

 for instance, certain crabs, such as the fiddler crab. Holmes 

 has found that these crustaceans also go sideways toward the 

 light. Jennings draws from this fact the following conclusions : 

 "The symmetrical position is an incident of the reaction, not 

 its essence." 



In other words, he uses these observations of Holmes to 



