ISOLATION 185 



believe, only through the possession of a distinctive call-note. 

 The cock-birds, who arrive first, must have some means of 

 attracting the hen-birds when they follow, and supposing that the 

 species frequents woods and not, like swallows, the bright seas 

 of air, success is easiest if they set to and sing their very loudest. 



If we investigate, we shall find that among European birds all 

 the great singers belong to species that are migratory over most 

 of their range though our comparatively warm winters make 

 them permanent residents in England. Even our nightingales, 

 garden-warblers, willow-wrens, chirfchaffs, cuckoos always 

 migrate, our blackcaps nearly always. Many robins, black- 

 birds, thrushes, wrens remain with us throughout the winter, 

 yet we must rank them among species that are partially 

 migratory. Their Scandinavian kin, to a bird, move south- 

 ward in autumn. 



Thus there seems to be a connection between migratory habits 

 and power of song, at any rate in cases where the bird is small 

 and of a retiring habit. Obviously the greater the likelihood of 

 the units of a species becoming scattered, the greater the need of 

 some means of bringing them together. 



I have been speaking of song as a call-note. It is, of course, 

 a secondary male characteristic and it is used to attract and 

 charm the hen. But song, though a special privilege of the male 

 and though it plays an important part in sexual selection, is, 

 nevertheless, the most powerful call-note of the species, the 

 shibboleth that marks them off from their nearest of kin. With- 

 out such a shibboleth how could the chiffchafFs and willow-wrens 

 after spending the winter together, when spring comes, pair 

 according to species ? 



II 



ISOLATION OF SPECIES OF FLOWERING PLANTS 



Except when autumn drapes the woods in red and gold, 

 nearly all the variety of colour that plants can boast is due 

 to their flowers. There is no limit to richness of the hue. 



