i2o THE POPULAR SCIENCE MONTHLY 



the tube-worms they thereupon draw back very quickly. Since their 

 locomotor organs are not symmetrical, but are arranged in a peculiar 

 unsymmetrical manner, they do not, after the next progressive move- 

 ment return to the former direction of movement, but deviate sideways 

 from it, and it is, therefore, easy to understand that such animals do 

 not furnish the best material to demonstrate the laws of heliotropism, 

 especially since they possess, moreover, only a slight photochemical sen- 

 sitiveness. But Jennings has with special preference used observations 

 on such organisms to argue against the theory of tropisms, and he has 

 with these arguments caused much confusion in the minds of zoologists. 

 One writer has, if I am not mistaken, asserted that the significance of 

 tropisms is limited by the demonstration of the sense of difference. 

 This writer overlooks the fact that it is a question of tracing psychical 

 phenomena, and not merely tropisms, back to physico-chemical processes. 

 Just as in muscles and nerves the action of a constant current is 

 different from that of an intermittent current, so we find in the action 

 of light an analogous case. If we wish to trace all animal reactions 

 back to physico-chemical laws we must take into consideration besides 

 the tropisms not only the facts of the sense of difference, but also all 

 other facts which exert an influence upon the reactions. The influence 

 of that mechanism which we call " associative memory " also belongs in 

 this category, but we can not discuss that further here. Instead the 

 reader is referred to my aforementioned book, as well as the newer work 

 of Bohn, " La naissance de l'intelligence." 18 Let us bear in mind 

 that " ideas " also can act, much as acids do for the heliotropism of cer- 

 tain animals, namely, to increase the sensitiveness to certain stimuli, 

 and thus can lead to tropism-like movements or actions directed toward 

 a goal. 19 



IX 



Besides light and the electric current, the force of gravity also has 

 an orienting influence upon a number of animals. The majority of 

 such animals are forced to turn their heads away from the center of 

 the earth and to creep upward. It was uncertain for a long time how 

 the orientation of cells in relation to the center of gravity of the earth 

 could influence the rate of the chemical reactions within, but it has 

 been suggested that an enlargement or shifting of the reacting surfaces 

 formed the essential connecting link. If it is assumed that in such 

 geotropically sensitive cells two phases (for instance, two fluid sub- 

 stances which are not at all, or not easily, miscible, or one solid and one 

 fluid substance) of different specific gravities are present, which react 

 upon one another a reaction takes place at the surfaces of contact. Every 



18 ' ' Comparative Physiology of the Brain and Comparative Psychology, ' ' 

 New York and London, 1900. 



19 Paris, " Bibliotheque de Philosophic scientifique, " 1909. 



