110 STUDIES IN GENERAL PHYSIOLOGY 
was of the opinion that the animal behaves thus through 
“instinct.” Ifa physicist finds that liquids rise in a capil- 
lary, or that one liquid forms a convex while another a concave, 
meniscus ina 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 its 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 larve 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 
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