1 84 PROBLEMS OF EVOLUTION 



a knowledge of their various notes (and each has several) 

 requires much time and patience, and may, even given leisure 

 and patience, end in failure. When I have explained my views 

 on the alternative nature of call-notes and distinctive plumage to 

 ornithologists, they have, as a rule, been of opinion that the case 

 for it is not very strong, one (a very advanced student of birds' 

 song) maintaining that all the notes of all the Tits are easily 

 distinguishable to him and, therefore, that each species can have 

 no difficulty in knowing its own from the rest. I cannot help 

 doubting the inference, especially as many birds (belonging most 

 of them, it is true, to more stupid families) are taken in by crude 

 imitations of their own notes. Among birds with decidedly 

 marked plumage and not very great song power may be 

 mentioned some of the Wheatears (Saxico/a), the Wagtails 

 (Motaci/Ia), the Shrikes (Lanius), Bullfinches (Pyrrhula) (until 

 taught a Bullfinch is no songster). Surely their distinctive 

 plumage has its use. It is sometimes argued that members of 

 a species know each other by " instinct," i.e., I suppose by the 

 aid of some sense which we do not share with them. But when 

 the two senses of sight and hearing are there, highly developed 

 and available for recognition, it is superfluous to imagine another 

 sense existing for the purpose. 



Vocal I wish now to point out the connection between vocal power 

 an( * mi g rat i n - ^ n many species (the cranes, for instance) the 

 loud ringing cries are useful for keeping the trailing flock 

 together. But small birds seem to migrate in silence and the 

 great displays of voice do not take place till their summer home 

 is reached. Herr Gathe told me that he had never heard the 

 song of the nightingale, and yet during the spring migration the 

 nightingale is common enough in Heligoland. During the 

 winter allied species often live together, willow-wrens with 

 chirFchaffs, meadow-pipits with rock-pipits and very possibly 

 with Tree-pipits. In Egypt, in January, I only once heard the 

 note of the chifFchafF; in Algeria, near Constantine, in the same 

 month, the air rang with their cries. But whether they are 

 dumb or not in winter, when spring comes, a separation must be 

 made and this separation must be possible in many cases, I 



