642 PHOTOPERIODISM IN VERTEBRATES 



spring that normally induces the onset of the breeding season of the 

 junco, and those who held that the stimulus obtained is in fact the 

 result of an artificial laboratory technique which in no way can be 

 taken as reflecting normal events in the wild. Nowadays there seems 

 little reason to doubt that the gonad-stimulating effects of artificial light 

 in autumn must be regarded as an exact parallel of those events which 

 take place naturally every spring. 



Recently, however, the importance of the day length as such has 

 been questioned, and attention has been focused on the possible direct 

 stimulatory effect of a reduced dark period (Jenner and Engels, 1952; 

 Kirkpatrick and Leopold, 1952). It has been shown in the case of 

 certain birds that a strong stimulus to gonad growth can be obtained 

 in winter conditions if, without altering the number of hours of light 

 per day, the long night is split into two shorter dark periods. It has 

 therefore been concluded that the total number of hours of light re- 

 ceived each day is unimportant, that the duration of the dark period is 

 critical, and that our theories regarding vertebrate photostimulation 

 must be amended. These conclusions have led to controversy, a num- 

 ber of authors including Farner et al. (1953), Hammond (1953), 

 and Marshall (1955) having expressed scepticism that the dark period 

 itself has any active role to play. 



However, the results obtained do need some explanation, and it 

 might be interesting to make the following speculation. It is possible 

 that, except during any refractory period, daylight always exerts a 

 stimulus to the production of gonadotropins in the birds in question, 

 but that, if the night is too long, the glandular activity is depressed to 

 such a degree that the succeeding short day is powerless to counteract 

 it. A splitting of the night period might be all that is required to 

 counteract the development of such an extreme depression and so per- 

 mit the light stimulus to be felt. 



This discussion has so far taken no account of species which do not 

 breed in the spring or which are not diurnal, and to which conse- 

 quently the same arguments cannot apply. Regarding nocturnal or 

 subterranean animals there appears to be no reliable evidence at all, 

 although of course spring-breeding nocturnal species may feel the 

 effects of longer days in terms of shorter nights in the manner men- 



