24 VERTEBRATE REPRODUCTIVE CYCLES 



3. ENVIRONMENTAL FACTORS 



Having reached this conclusion it becomes necessary 

 to attempt to analyse the environment in which the 

 animals live, and to distinguish those of its many com- 

 ponents which influence the sexual cycle. Of course, 

 since some species are to be found breeding regularly in 

 each season, it can be guessed at once that the same 

 environmental factor is not operative in all cases. Among 

 spring-breeding animals it is possible that an increase 

 in the clemency of the weather, in daylength, in temper- 

 ature, and in the quality and quantity of the food 

 available may one and all be important, but in the 

 case of autumn-breeding species it appears necessary to 

 think in the opposite terms. In some cases, too, the moon 

 may be suspected of exerting an influence, and in some 

 parts of the world the seasonal rains may be of im- 

 portance. 



Of all these possible factors it seems true to say that 

 until a generation ago only food and temperature were 

 thought to be important, and this was perhaps due to the 

 fact that man himself notices these more than he does 

 any of the others. Certainly the food and temperature 

 theories were widely held, and they can be seen to have 

 influenced the writings of Heape^*^' about 1900 and of 

 Zuckerman202 as lately as 1932. Even to-day, when they 

 have been largely discredited, it must be remembered 

 that breeding cannot be successful unless animals have 

 suitable food and a proper degree of warmth, particularly 

 if they are cold blooded. 



Nowadays, being wise after the event, it appears 

 surprising that so little attention was given to the ques- 

 tion of daylength, particularly in temperate regions. If 

 one had consciously to choose some environmental 

 variant by means of which a particular season of the 

 year could be regularly observed, it would soon become 

 obvious that nothing else could give such certain results. 

 A spring may be warm or cold, the food supply may vary 



