THE ENVIRONMENT AND REPRODUCTION 43 



the psychological (i.e. nervous) stimulation of a physio- 

 logical mechanism. Summing up, it may be said that 

 unless all those factors which constitute an animal's 

 immediate environment are felt to be satisfactory, then 

 the purely physiological mechanism of reproduction is 

 inadequate to ensure complete maturity and successful 

 fertilization, so that the production of eggs or young 

 becomes impossible. 



II. CONCLUSIONS 



The breeding seasons of most vertebrate species are 

 determined by tw^o sets of causes, which have been called 

 by Baker 10 the ultimate and the proximate. The ultimate 

 causes of the timing of a breeding season are the reasons 

 why that time of the year is the most beneficial for this 

 purpose. Probably the most important single reason is 

 that the food supply should be adequate both in quantity 

 and quality to satisfy the extra needs of the young when 

 they appear. As to the mechanism by which such 

 breeding seasons have become genetically fixed, it is 

 generally supposed that natural selection must have 

 been responsible. 



The proximate causes which determine the timing of a 

 breeding season are three-fold. First there is the internal- 

 reproductive rhythm, which usually has an approximate 

 periodicity of one year. Such a rhythm may be stronger 

 or weaker according to the species and also according to 

 age and season, but that it exists generally there can be 

 very little doubt. The results of experiments such as 

 those with minnows, which when kept in the dark in 

 spring nevertheless come to maturity, are conclusive on 

 this point. 



The second of the proximate causes which determine 

 the time of breeding is that environmental variant (sun, 

 moon, rain, etc.) to which the cycle is attuned. These 

 factors have been listed in detail, and it has been stressed 

 that their function is to ensure an accurate timing of the 



