CHAPTER III 



THE ENVIRONMENT AND REPRODUCTION 



I. THE NATURE OF SEASONAL CYCLES 



IT has been stressed that animals tend to breed only at 

 certain times of the year, and the question which now 

 arises is whether the timing of the yearly sexual cycle is 

 achieved by means of an internal physiological mechan- 

 ism, or whether it is imposed from outside by the 

 influence of the seasons themselves. 



A preliminary answer to this question may be given as 

 follows. If the control of the cycle is solely internal then 

 the timing of the physiological mechanism must be as 

 good as that of a perfect clock, and since such a degree 

 of precision is unknown in any biological process,' this 

 appears to be inconceivable. Although there is good 

 evidence that internal physiological rhythms do exist, 

 there is, as Baker i* has said, *no possibility that annual 

 phenomena can be wholly controlled by them in any 

 species of plant or animal , and it is therefore necessary 

 to suggest that some factor in the environment period- 

 ically 'puts the clock right'. 



2. EVIDENCE FROM THE MOVEMENTS OF ANIMALS 



With the question of an internal rhythm momentarily 

 set aside, it is relatively easy to build a prima facie case 

 for the control of yearly reproductive cycles by seasonal 

 changes in the environment. In the first place there is 

 the argument of reductio ad absurdum set out above, and 

 in the second there is the evidence provided by the 

 results of animal movements. On this latter point 

 text-figures i and 2 may be referred to again. These 



