BIRDS 113 



north. In this way, from the fact of each succeeding 

 generation adding a piece, however small, to the original 

 journey, we get in the course of an enormous period the 

 gigantic travels of our actual migratory birds. And 

 as the route is only a little longer in each generation, 

 selection does not make too exacting a demand on the 

 birds, and there will always be some that can perform 

 the longer journey. 



Thus the power of flight has been strengthened 

 gradually, and the sense of direction has steadily 

 increased up to its present pitch. This sense consists 

 especially in a marvellous memory, which has grown 

 steadily as the route lengthened. If we did not know 

 that it had been built up gradually, we should hardly be 

 able to understand to-day how the birds can retain an 

 impression of the routes they have only travelled over 

 twice with their parents. But this memory is not so 

 much a capacity for observing the path over the regions 

 they have traversed as the power to keep the various 

 directions that they took in their flight, and so there 

 must also be a sense of orientation, or direction, that 

 enables the birds to keep their way even when they are 

 turned aside. That they do not need to any great 

 extent to look down on the country is clear from the 

 fact that night-time is often chosen for the migration. 

 Nevertheless, they do need some sight of the earth, 

 however faint and shadowy, otherwise they would rise 

 above the clouds, and would not fly so low in bad 

 weather. 



However, this enormous development of memory in 

 the migratory birds is not altogether strange. Even 



