284 W. C. ALLEE . , 



tested from time to time. Thus the last trials recorded in part 2 

 of the table were all made on the same group of animals, with the 

 exception of two cases the oxygen content of the water was high 

 and in all cases the amount of carbon dioxide present as free car- 

 bon dioxide was very small, so that these changes in reaction are 

 not due to a modified gas content of the water. 



It will be seen in later experiments, that the degree of positive- 

 ness depends upon the metabolic rate of the animals. That 

 is, in animals having a high rate of metabolism there is a high 

 positive response. From this point of view the decrease in the 

 positive responses w5uld be accounted for by assuming that the 

 animals are in a state of lowered metabolism during the breeding 

 season. This view is supported by those plants and lower animals 

 that reproduce asexually during conditions favorable for growth 

 and respond to poorer external conditions by sexual reproduction. 



The cause of the variation in response is not impossible of solu- 

 tion although at present it cannot be treated entirely from the 

 experimental side. Fiom the results given it is evident that 

 during the breeding season not all isopods are in the same physio- 

 logical condition at any one time. The results from any one day 

 as listed in the table show wide variations, yet these were taken 

 from almost identical external conditions, so the variable quantity 

 in this case must be an internal one. This view is further sup- 

 ported by comparing those results where the animals were in the 

 copulating position. This term is used in the table to show when 

 the females were being carried by the males. There are three 

 cases given when all the individuals tried were in this position. Of 

 these one gave a positive response of 11 per cent, another of 70 

 per cent, and the third gave no positive response at all. The 

 first were taken from a very low oxygen content and as will be 

 seen later this tends to decrease the positive response. Yet 

 with this added complication they gave a much higher response 

 than did two pairs taken the same day from another place in the 

 same stream. It may be that in the case where no positive re- 

 sponse was given the animals were near the actual copulation time, 

 and the animals giving the 70 per cent response may have been 

 far from this period. 



