ENVIRONMENT, PAST AND PRESENT 57 



to have been totally ignored by students of migra- 

 tion. It has been the subject of investigation at 

 Edmonton for some years. The further from the 

 equator one goes in either hemisphere, the less ultra- 

 violet radiation reaches the earth's surface. This 

 applies mainly to the winter but not entirely so, for 

 owing to the angle of the sun in the extreme north, 

 even at midsummer there is material reduction, by 

 atmospheric absorption, of the available ultra- 

 violet. In the latitude of Edmonton there is prac- 

 tically no effective ultra-violet radiation for at least 

 the four winter months when the sun is at its lowest. 

 Further north still this deficient period is greatly 

 extended. Even at the United States boundary the 

 winter months can provide but inadequate radia- 

 tion. These short-wave rays belong to the non- 

 visible series, i.e. although they can penetrate the 

 lens and cornea and so actually reach the retina, 

 they do not give rise in the human eye to the sensa- 

 tion of light. Their particular interest lies in their 

 power to produce the substance known as vitamin D 

 by direct action on certain other substances and 

 vitamin D is essential to the welfare of the animal 

 organisation, whether young or adult. This vita- 

 min occurs in various foods such as cod-liver oil and 

 a sufficiency may be obtained from sources of this 

 kind to meet all requirement in regions where there 



