EVOLUTION 



53 



Fig. 10 



Distribution of colour-phases of the arctic skua, Stercorarius parasiticus, from 

 H. N. Southern (1943), showing isoHnes for percentage of the pale phase 



in the breeding population. 



environmental factors. In the Atlantic (though not the Pacific) part of 

 the fulmar's range the situation is in many ways the opposite of that 

 among the arctic skuas; where the surface of the sea is above freezing 

 (i.e. in the Low Arctic and rest of the range) the fulmars are nearly all 

 light. The gradient runs from o per cent, dark in Britain to a hundred 



