44 VERTEBRATE REPRODUCTIVE CYCLES 



sexual season. Normally the stimulus of the external 

 factor reinforces that of the internal rhythm, but if by 

 experiment they are made to clash then either one or the 

 other may prevail. Usually the external factor is the more 

 powerful and over-rides the influence of the internal 

 rhythm. 



The third of the proximate causes is that group of 

 factors in the immediate neighbourhood of the individual 

 animal (the breeding area, the mate, and perhaps the 

 social group), which must also conform to an appropriate 

 pattern before the final act, the shedding of the eggs and 

 sperm, becomes possible. Perhaps it may be objected 

 that this last distinction has been too sharply drawn. The 

 so-called environmental factors and the so-called 

 psychological factors may in fact merge together so that 

 the effect of the moon on ovulation in the nightjar, or of 

 water on the spawning of a desert frog, may be more 

 in the nature of a psychological stimulus. As has been 

 repeatedly stressed much further work on such problems 

 is needed. 



