ANNUAL MIGRATIONS 115 



migration, but they cannot be the fundamental 

 stimulus. Migrations would be occurring at all 

 times of the year were this the case. 



Change of color in the leaves of trees has been 

 suggested. It has been experimentally shown that 

 birds can appreciate the yellow end of the spectrum 

 and from this viewpoint there is no difficulty. But 

 birds that should leave us in September, let us say, 

 would find themselves in a serious quandary in the 

 years that have experienced early frosts and have 

 witnessed the falling of the leaves in August. July 

 migrants have probably never in their lives seen the 

 autumnal tints of the northern woodlands. This 

 suggestion seems to be wholly untenable. 



Ultra-violet radiation offers a factor of the envi- 

 ronment that is presumably, though by no means 

 certainly, stable but we know of no mechanism 

 whereby birds could appreciate its seasonal varia- 

 tions with sufficient promptness or intensity to 

 render them effective as stimuli. 



The one factor of the northern environment that 

 is thoroughly dependable and certainly without 

 fluctuation is the seasonal variation in day-length. 

 Daylight (with its sunshine) is of vital importance 

 to most members of the animal kingdom and exerts 

 a profound effect on life generally. It has induced 

 curious rhythms in living protoplasm from bacteria 



