296 THE BIOLOGY OF THE SEASONS 



How is it that a swallow which has wintered in Africa 

 comes back to its birthplace in the South of England ? 

 How is it that young birds, who never left the parish before, 

 start off on a Summer evening and make their way to the 

 Nile Valley ? How do they keep their direction in the 

 dark and at great heights, or do they very often go very 

 badly astray ? If it be true that the northward-bound 

 Spring migrants often take a short cut, different from their 

 Autumn journey south, how is this to be thought of ? 

 Sailing northwards from the Cape by the West Coast route 

 one meets in Autumn numerous migrants making their 

 way south. Many of these rest for hours on the ship, but 

 most of them certainly continue their southward journey. 

 Why? 



a. It has been suggested that success in way-finding 

 may be due to inherited experience, slowly cumulative 

 from generation to generation, enriched and specialised 

 by individually minute contributions. There are two great 

 difficulties in the way of this theory. In the first place, we 

 have no secure warrant for believing in the direct ent ail- 

 ment of the lessons of experience. In the second place, it is 

 difficult to see what content the experience could have in the 

 case of birds flying by night, and often at great heights, as 

 so many do. 



b. A second theory attributes the success of the 

 migratory effort to tradition, and there may be some truth 

 in this attractive suggestion. The idea is that those lead 

 well one year who followed well for several years before. 

 There may be in each rushing troop some old, experienced 

 guides. Again, however, there are difficulties. Every 

 individual experience must have a content of definite and 

 concrete impressions, but what precise experience is to be 

 had out of a night journey at a high altitude over the 

 pathless sea? If we begin by traversing a difficult and 



