118 JEAN DAWSON 



ment so varies in members of the species and in the same in- 

 dividual from time to time that no general statement can be 

 made unless the physiological state is known. 



The physiological state of Physa varies with its experience; 

 and its experience varies with the environment. The greater 

 variety of experience to which it is accustomed, the more readily 

 it may be approached without producing the physical reactions 

 which are a correlate to the physical state of fear in the higher 

 animals, and the more readily it is tamed. A physiological 

 state in Physa can be transposed by transposing the conditions 

 which produce it. A new experience produces a shock to Physa 

 which seems to deprive it temporarily of its sense of gravity. 

 The snail becomes accustomed to the experience, if it is repeated 

 eight or ten times within an hour and a half; so that it reacts 

 to it as do other snails to which the experience is familiar. 



By the use of the siphon, Physa shows that it distinguishes 

 changes in its environment. By a process of association it 

 "remembers" the location of the film, the character of the 

 sides of the aquarium and even the relative depth of the water. 

 It also shows the development of neural habit. 



The young are much less sensitive to stimuli of their environ- 

 ment. Very young snails show none of the reactions which 

 are the correlate to the physical state of fear in the higher 

 animals, but these quite suddenly disappear when the animal 

 is about five mm. in diameter. The sudden appearance of these 

 reactions is due, perhaps, to a rapid development of the nervous 

 system (Donaldson, 1898), rather than to a sudden appearance 

 of a delayed instinct, as Herrick (1901) believes to be true for 

 birds; since there appear at the same time associations of the 

 habitat. 



The tapping reaction of the young is the same as that of 

 the older snails, when it takes air at the film. This reaction 

 changes as the snail grows, and is modified into the better 

 adapted one of the adult. 



