g6 DARWINISM AND THE PROBLEMS OF LIFE 



to detect their fellows in the wide forest. The mammal 

 has an easier task. He follows the track of his 

 companion with his nose ; but the air leaves no 

 tracks. 



In all these cases sight is of no use in detecting the 

 species ; it has to be aided by another sense — hearing. 

 The voice is developed as a means of recognising 

 the species. But it not only serves the purpose of 

 bringing the sexes together ; it is of the greatest 

 importance in the common life of the bird. We know 

 how powerful the social instinct is in birds, and we 

 have learned to appreciate the value of this gregarious 

 sense for the maintenance of the species. Apart from 

 the fact that the young learn continually from the 

 older, they are also protected by them from danger. 

 Every sportsman knows that when he is approaching 

 a flock of rooks, and one member of the flock sees him, 

 it is useless to go any farther. The understanding 

 between the flock can only be brought about by voice. 

 In the social life of many animals it is extremely 

 important to specialise the call, as it is an immense 

 advantage to the species if the bird can produce different 

 notes. In that case one note will serve to bring the 

 flock together, another to warn them of approaching 

 danger, a third may be the signal for flight. We are, 

 in fact, astonished at the diverse cries of the crows, and 

 the different notes of the blackbird, which is often a 

 herald of warning even to other animals in the forest. 

 Nearly all migratory birds have a peculiar call on their 

 long, nocturnal travels, in which some means of keeping 

 ' together is necessary. But it is especially the pairing- 



