ENVIRONMENT, PAST AND PRESENT 63 



may be as essential as a steady supply of insects 

 is to the insectivorous species. 



It is interesting to note in passing that juncos and 

 other migratory seed-eaters released from my avia- 

 ries during mid-winter, with the ground under snow, 

 have been out for as long as two weeks before being 

 retaken and have fared so well on the vacant lots 

 that they have not even bothered to go to the food- 

 box on their return. The last of our liberated crows 

 to be shot was killed on February 28, 1930, in the 

 northern wilderness, after having been free for four 

 months during a quite severe winter. Wild indi- 

 viduals of various seed-eating species that have 

 failed to migrate, frequently survive the entire 

 winter. Such isolated examples do not, of course, 

 prove that the race generally could find a sufficiency 

 of food, but they distinctly suggest that such might 

 be the case, and automatically eliminate dogmatic 

 assertions to the contrary. 



This viewpoint may be summarised in the follow- 

 ing manner. Assuming that vitamin D (like 

 vitamin A) is indispensable to the health of adult 

 animal life then inhabitants of the north must get 

 their supply either from their diet or from exposure 

 to the sun or from a combination of both. As- 

 suming that their diet is such that they require a 

 certain amount of sunshine per annum then, to re- 



