722 



THE ORIGIN OF SPECIES 



forms resemble each other more than they do those of the inter- 

 vening continent. Where separate forms cannot be recognised 

 there is often a chne, that is, a gradual change either in the 

 magnitude of some measurable character, such as size, or in the 

 frequency of occurrence of a particular character. An example 

 of the second type is given by the change in the percentage of 



Fig. 553. — Heads of bridled (lower) and normal varieties of guillemot. 



the bridled form (Fig. 553) of the guillemot {Una aalge) present 

 in the breeding colonies as one goes from south to north. This 

 type, which has a white line running back from the eye, is not 

 present in Portugal, makes about i per cent, of the birds in the 

 English and Bristol Channels, and then becomes fairly steadily 

 commoner until there are from 10 to 16 per cent, round the 

 north coast of Scotland and in the Hebrides. Farther north still 

 it becomes even more important, and makes over 50 per cent. 

 of the birds in Iceland (Fig. 554). 



