THE ENVIRONMENT AND REPRODUCTION 39 



The next environmental factor to be considered is that 

 of temperature. Among the warm-blooded birds and 

 mammals it is not known to play any great part in stimu- 

 lating or preventing breeding, although of course it has 

 a potent effect on food supply, particularly in the case 

 of insectivorous species. It is this lack of food, and not 

 the cold as such, which is the ultimate cause of the 

 migration of so many birds from arctic and temperate 

 regions in the autumn. If one of these birds is kept back 

 by force and its food supply artificially assured then, as in 

 the case of Rowan's juncos,i6o it may survive and be 

 brought into breeding condition in the most extreme 

 arctic conditions. Of course this may not be possible 

 with all species, and certainly birds such as the canary, 

 which come from warmer climates, cannot be stimulated 

 to breeding condition during a Canadian winter. i^ 5 



In cold-blooded vertebrates, the fishes, amphibians, 

 and reptiles, the importance of temperature is obviously 

 great. Towards freezing-point the metabolism of most of 

 these animals is so far slowed down that they become 

 sluggish or even immobile, and in such conditions 

 breeding would clearly be impossible. Animals such as 

 these are perhaps more likely to make use of the rising 

 temperatures of spring to regulate their breeding 

 seasons. Such a mechanism may perhaps be common 

 among those sea fishes which breed in spring and 

 summer because the temperature of the sea varies 

 steadily and is not subject, like that of shallow fresh- 

 water, to violent and unseasonable fluctuations. Certainly 

 the marine and brackish water killifish reacts to an in- 

 creasing temperature by the development and matura- 

 tion of its testes, 60 and there is some evidence that the 

 same may be true of the fresh water perch^^^ and of the 

 three-spined stickleback.'^ Among the amphibians of 

 temperate regions the temperature of the environment 

 may also be a critical factor. This seems to be true of the 

 edible frog^* and of the American newt, Triton virides- 



