60 THE RIDDLE OF MIGRATION 



may survive for as long as three years but every 

 March sees some of them developing fits and suc- 

 cumbing. Nothing of the kind has been noted with 

 juncos but those that have attempted to breed in 

 the aviaries have never yet done so successfully. 

 They are close sitters but their eggs nevertheless 

 have always failed to hatch out. Generally the 

 chicks develop almost to the point of hatching but 

 they never leave the shell. The eggs have been 

 transferred to canaries, but the outcome is the 

 same. This certainly suggests vitamin D deficiency .^ 



Quite apart from their connection with vitamin 

 D, the ultraviolet rays are in themselves of direct 

 therapeutic value. 



There is another phenomenon as characteristic of 

 the northern sections of the globe as the migration 

 of birds. This is the periodic fluctuations in the 

 numbers of resident animals. The ten-year rabbit 

 cycle of Canada is universally familiar. It occurs 

 also in the case of numerous other mammals and 

 of many resident birds. The animals concerned 

 slowly increase in numbers till they reach a maximum. 



2 The anti-infection, or xerophthalmic, vitamin A is particular!}- 

 associated with resistance to disease but it is no doubt true of all 

 the vitamins that deficiency results not only in. specifically asso- 

 ciated ailments but in a lowering of general health and vitality 

 and hence resistance to disease. 



