564 RAYMOND PEARL. 



movement, and its general tonus becomes lower and lower. 

 As a result of this it must remain relaxed for a certain time 

 in order that recovery may take place. When in the course 

 of the activity of the animal its general tonus gets below a 

 certain point it stops, the actual process of coming to rest 

 being a more or less gradual one. A strong piece of 

 evidence in favour of this view is the fact already given in 

 the section on " Natural History/' namely, that if the 

 animal is stirred up and made to start moving again im- 

 mediately after coming to rest each time, it will be found 

 that the periods of activity become progressively shorter. 

 Furthermore, when the general physiological condition of 

 the organisms is weakened by keeping them for a time in 

 the laboratory, it is found that the periods of rest become 

 progressively longer in proportion to the periods of activity. 

 The general " predisposing condition " to the coming to rest 

 is then probably a lower tonus due to fatigue. The im- 

 mediate causes determining the exact place chosen are of 

 three sorts. First, and probably most important of these, is 

 the intensity of the light. It is well known that planarians 

 tend to come to rest in regions of comparatively low light 

 intensity, the reaction having been first noted by Loeb ('93), 

 and called by him " Unterschiedsempfiudlichkeit." This 

 factor seems to bo the most important of any in determining 

 the region in which the animals come to rest, both under 

 experimental conditions and in the natural habitat. In 

 aquarium dishes placed close to a window, and containing 

 considerable plant material, the worms will be found resting 

 practically always in the half of the dish away from the 

 window. The largest number of individuals will be 

 entangled in the plant material, and usually for the most 

 part invisible ; while of those specimens resting on the sides 

 and bottom of the dish the greatest number will be found in 

 such places that there are heavy masses of plant material 

 between them and the window. A few will come to rest far 

 around on the sides of the dish where the glass itself cuts off 

 some of the light. This last position has been mentioned by 



