CAUSE AND ORIGIN OF MIGRATION 29 



north, and to birds of the same species which winter 

 in La Plata, arriving from supposed breeding places 

 to the south when the northern birds leave. Cap- 

 tain R. Crawshay, author of : The Birds of Tierra 

 del Fuego/' found it in this little known land, but 

 speaks somewhat doubtfully of its identity ; we 

 shall probably learn that the southern form is sub- 

 specifically distinct from the northern. There are 

 other wide-ranging waders which are suspected of 

 having a southern nesting area, but we still await 

 proof. 



The lack of sufficient or suitable food in the winter 

 home during our northern summer may also cause 

 the exodus, but this is a difficult point to prove when 

 it is remembered that the winter home of every bird 

 is not the parched tropical land or the waterless 

 desert. From some zones removal must be a neces- 

 sity, but in others there is food for all, so far as man 

 can tell. 



Dr J. A. Allen, a severe but discriminating critic 

 of migration theorists, says- ' Migration is the only 

 manner in which a zoological vacuum in a country 

 whose life-supporting capacity is a regular fluctuat- 

 ing quantity, can be filled by non-hibernating 

 animals" (51). When in the early days of migra- 

 tion this periodically-supplied northern zoological 

 vacuum was filled to overflowing by the increased 

 numbers of avian inhabitants at the close of the 



