AVIAN MIGRATION AND REPRODUCTION 689 



and nature of the preceding photoperiod. Rate of testicular response 

 was interpreted as a direct function of the summated daily gonado- 

 tropic effects of the photoperiods and the carry-over periods. The 

 interpretation of Farner et al. is in agreement with the hypothesis of 

 summation of daily physiological increments, but they suggested the 

 carry-over period as an additional effective part of the photoperiodic 



cycle. 



Digressing for a moment, I should like to point out in relation to 

 this problem of the role of light and darkness that although the work 

 in our laboratory showed clearly a relation between rate of response 

 and the daily photoperiod, the experiments were not designed to 

 distinguish between the roles of the photoperiod, the dark period, or 

 a relation between them. The summation hypothesis was stated initially 

 in terms of the response to day lengths, or photoperiods, but it was 

 pointed out later that these terms were used only relatively. The role 

 of the dark period in the daily schedule will be discussed shortly, but 

 I wish to distinguish here between the theory of summation of daily 

 increments of physiological change in response to an effective photo- 

 periodic schedule, over which there is no disagreement, and the role 

 of light and darkness in the daily photoperiodic cycle, over which 

 there is disagreement. The theory of summation, it must also be 

 pointed out, is only an interpretation at present; there is no objective 

 evidence for it. 



The results of the two experiments just reported were interpreted 

 as demonstrating an inhibitory effect of a long dark period. But they 

 also showed that the ratio of the durations of the light and dark 

 periods might also be a factor. The next experiment was designed to 

 test the role of the ratio of light to darkness and to obtain data bear- 

 ing on the existence of a carry-over period. The schedule chosen was 

 1L-2D. In each cycle there was the same proportion of light to dark- 

 ness and the same total amounts as in a schedule of 8L-16D, which 

 is not stimulatory. If the birds failed to respond, then it would seem 

 unlikely that there was an effective carry-over period. The birds 

 showed an excellent and rapid gonadal response. The results demon- 

 strated again that only a small total amount of light per day, in this 

 case 8 hr, is highly effective when there is no long inhibitory period 

 of darkness. Since the proportion of light to darkness in each cycle 



