30 VERTEBRATE REPRODUCTIVE CYCLES 



5. LIMITATIONS OF THE LIGHT CONTROL THEORY 



With all the popularity of a new and stimulating idea, 

 the light theory of the control of vertebrate reproductive 

 cycles gained ground so rapidly during the period 1926 

 to 1939 that its application was carried too far by many 

 of those who discussed it. A reaction to this state of 

 affairs also developed rapidly. In 1930 Hill and Parkes^^^ 

 were already pointing out that the ferret in domestication 

 has a well marked breeding season from April to August, 

 and were finding it 'difficult to imagine what environ- 

 mental changes take place during April in an artificially 

 heated animal house not particularly accessible to direct 

 sunlight'. In 1934 when they discovered that their 

 ferrets did not experience infertility after a reduction of 

 daylength in winter and early spring, they concluded 

 that the onset of the breeding season is not dependent 

 on the increasing length of day.^^^ They even went 

 so far as to suggest that the provision of extra light in 

 winter is an artificial laboratory technique, the results 

 of which do not reflect conditions in nature, and that the 

 breeding cycle is solely dependent on an internal 

 physiological rhythm. 



The impossibility of such a rhythm being the sole 

 factor in control of breeding seasons has since been 

 recognized,!* but further criticisms were also advanced. 

 The first of these referred to the problem of nocturnal and 

 subterranean animals which, it would seem, could not be 

 affected by light. The second was the problem of those 

 animals which normally breed in autumn and winter at 

 times of decreasing or of minimum daylength. The 

 third was that of the tropics where the daylength is 

 relatively constant at all times. 



As regards the first of these problems there is still no 

 information, probably because of the practical difficulty 

 of maintaining nocturnal or subterranean animals for 

 any length of time in the laboratory. The question of 

 autumn breeding has, however, been investigated, and 



