ENVIRONMENT, PAST AND PRESENT 59 



hemisphere when they are reared on a deficient diet. 

 Seed-eating birds avoid rickets in their young by 

 becoming insectivorous entirely, or in part, during 

 the rearing period. Cereals are notoriously defi- 

 cient in vitamin D; insects form a rich source. 

 Merlins and other hawks, which normally feed their 

 young on plucked birds, a diet that induces rickets, 

 meet the situation by periodically administering 

 feathers to them, an effective cure for rickets also 

 in meat-raised, hand-reared hawks. In adults the 

 symptoms are not so obvious. An interesting dis- 

 covery, made at the University of Toronto, is that 

 adult rats kept on a diet deficient in vitamin D and 

 sheltered from sunlight speedily succumb to infec- 

 tion from specific organisms, the resistance of rats on 

 a similar diet but exposed to sunshine (or artificial 

 radiation) being 50% to 70% greater. Lack of 

 vitality in embryos of the fowl, when the hens have 

 been laying while on a deficient diet, has also been 

 demonstrated. Exposure of the laying birds to 

 ultra-violet radiation cures the situation. In vari- 

 ous other ways the essential nature of vitamin D 

 to adult animals has been shown. 



At Edmonton we have kept tree sparrows and 

 j uncos for a period of years to ascertain the effects 

 of compulsory residence in the north with its ultra- 

 violet deficient winter. Individual tree sparrows 



