86 VICTOR E. SHELFORD. 



ceilings and activity began to alternate. This was followed by 

 heightened activity and stiffening as before. In the gradient a 

 negative reaction to air of more than a minimum evaporating 

 power was clearly shown as indicated by Table I., and Chart I., 

 Experiment n (p. 87). The salamanders appeared to sense 

 the drier air at once as indicated by hesitation or by turning back 

 when the change was encountered. The latter indicates that 

 these animals have a sense of orientation in the gradient. They 

 usually tried the driest air one or more times and the different 

 trials were usually followed by turning when it was encountered 

 again. They usually piled together in the moist air after 13 to 1 8 

 minutes and remained so for considerable periods. P. glutinosus 

 is clearly more sensitive to dryness than is P. cinereus. While 

 the former was clearly more stimulated in both observation tubes 

 and gradient experiments than was the latter, stiffening due to 

 drying appeared so early that in the gradient experiments 

 glutinosus did not turn back as definitely as did cinereus. 



The wood frog was stimulated at once in the dry air; it showed 

 agitation at first but very soon (a few seconds to five minutes) 

 crouched close to the bottom, drew the legs close to the body, 

 and partially or wholly withdrew and closed the eyes. After 

 this had continued for a time, the frog usually hopped in the 

 direction in which it was headed and if the disturbance was not 

 relieved it repeated the crouching. 



While the frogs sometimes appeared to orient in the gradient, 

 this capacity is poorly developed and the graphs are quite dif- 

 ferent from those of the salamanders. The frogs showed a 

 preference for the moist air and avoided the dry air mainly by 

 random hops and a lesser tendency to hop in the moist air. 

 The difference in the appearance of the animals in the different 

 parts of the experimental cages was striking. In the air of 

 highest and medium evaporating powers the withdrawal and 

 closing of the eyes just referred to took place to a degree ap- 

 parently proportional to the rate of evaporation. Here the 

 skin was dry and dull. In the moist air, the difference after a 

 few moments exposure, was striking. The skin glistened with 

 moisture, the frog sat upright, the eyes were fully protruded and 

 wide open, and the animals gave an impression of sagacity not 



