RHEOTAXIS, RESISTANCE TO POTASSIUM CYANIDE 411 



to about its old average and plays up and down as the metabolic 

 rate varies about its new mean. So the rheotactic reaction is an 

 expression, not of the absolute metabolic rate of the animal, but 

 of the relative metabolic conditions under which the isopod is 

 acclimated for the time being. Further experimental evidence 

 on this point will be published elsewhere in conjunction with 

 Dr. Shiro Tashiro. 



But even with this relative scale of metabolism determining the 

 rheotactic reaction of the isopod in place of an absolute scale, the 

 fact remains that individuals having a short survival-time usually 

 have a high rate of positiveness to a water current. This was 

 determined for individuals as follows: An assistant read me the 

 rheotactic reaction of isopods in groups of two from the same 

 stock, giving individuals that had at least 10 per cent of differ- 

 ence in their positive rheotactic response. Basing judgment on 

 the rheotactic reaction alone I was able to predict the relative 

 survival-time in 106 out of the 160 cases tried. This shows that 

 the rheotactic reaction in 66 per cent of the cases was a correct 

 indicator of the absolute metabolic conditions of the isopods 

 compared. Isopods with a highly negative reaction gave the 

 most difficulty, while it was impossible to predict the survival- 

 time of isopods that did not move at all in the current. The 

 explanation of this difficulty has already been given (pp. 405-406). 



Jennings ('06) clearly outlined the problems in this particular 

 field of animal behavior when he said, ^'We are compelled to 

 assume the existence of changing physiological states. This 

 assumption besides being logically necessary, is of course sup- 

 ported by much positive evidence drawn from diverse fields, and 

 there is reason to believe that in time we shall be able to study 

 these states directly." 



With the application of the cyanide method to problems of 

 animal behavior this prophecy is a step nearer its final fulfilment 

 and it is now possible to demonstrate directly as regards the 

 rheotactic reaction of isopods that high positiveness is the expres- 

 sion of a relatively high rate of metabolism, and low positive- 

 ness, of a low metabolic rate. 



December, 1913 



