110 STUDIES IN GENERAL PHYSIOLOGY 



was of the opinion that the animal behaves thus through 

 "instinct." If a physicist finds that liquids rise in a capil- 

 lary, or that one liquid forms a convex while another a concave, 

 meniscus in a glass tube, he will be less easily satisfied than the 

 zoologist, according to whom everything is done through 

 "instinct." The physicist will endeavor to discover more 

 precisely what conditions underlie the phenomenon. This, 

 it seems to me, is also the problem of the biologist a prob- 

 lem which is not even recognized, much less solved, by 

 saying the cause of such or such a motion is an "instinct." 

 From a biological standpoint one would at first take it for 

 granted that light causes animals to creep into crevices. 

 But I was able to show that the animals creep into the 

 crevices between solid bodies even when the solid bodies are 

 perfectly transparent and are exposed to a strong light; 

 secondly, that the animals behave in a similar way when 

 put in a perfectly dark room. Light is not, therefore, the 

 physical cause which determines this phenomenon. I proved 

 this for Forficula, ants, the larvae of Musca vomitoria, etc. 

 Plateau had previously established this fact by a similar 

 experiment upon Cryptops, with which I was not familiar at 

 that time, however. The animals creep into narrow crevices, 

 therefore, not because of the light, but because they are 

 forced to bring as much of their bodies as possible in contact 

 with solid bodies. The friction and the pressure produced 

 by the solid bodies are therefore the determining cause. This 

 view, that light has nothing to do with the phenomenon, but 

 that it is the friction produced by contact with solid bodies, 

 has this advantage over the traditional phrase " It is instinct," 

 that pressure and friction are physical agencies which, like 

 light, can be controlled quantitatively and qualitatively, and 

 by which we can prescribe unequivocally the "voluntary" 

 movements and the "voluntary" orientation of an animal. 

 I will here add that, while there are a large number of 



