xxv. s FLUCTUATIONS IN NUMBERS 665 



tions these factors, either alone or in association with cycles of solar 

 radiation or other climatic factors, would produce an oscillatory 

 system. 



When the numbers are at a maximum the animals show unusual 

 behaviour patterns, including migration, and may enter into a patho- 

 logical state, becoming cold and torpid and with a very low blood- 

 sugar content. Probably the pressure of competition and lack of food 

 operates on the hypophyso-adrenal system to produce a state of shock. 

 It is this, rather than disease or predators, that finally reduces the 

 numbers. 



Such phenomena are probably not peculiar to small mammals, but 

 appear markedly in them because the rapid breeding and short life 

 provide cycles of relatively short period. The facts are all consistent 

 with the interpretation already reached that the control of animal 

 populations depends very largely on the interaction of biotic factors. 

 This continual pressure of the animals on each other is probably 

 the main factor responsible for the great variations in the charac- 

 teristics of animal populations in different parts of their range, which 

 is found wherever sufficiently careful study is undertaken. This 

 variation in turn leads to the phenomenon of organic evolution and 

 the continual replacement of types by their descendants, which appears 

 so clearly as one follows the history of the vertebrates. 



