286 THE MIGRATION OF BRITISH BIRDS 



Geographical Dispersal hitherto attacked at the Wrong End 

 — Exterminating Influence of Glacial Epochs — Powers of 

 Organisms to Extend their Areas of Dispersal — This Dispersal 

 not Fortuitous but governed by Law. 



The subject of the present volume, the Migration and 

 Dispersal of British Birds, has been selected to illustrate 

 the development and application of what I believe to 

 be an entirely new Law governing the Distribution, 

 Emigration, or Dispersal of Species. In elucidating 

 this subject a very wide and varied series of phenomena 

 have had to be dealt with, yet no more than were 

 absolutely necessary to furnish a satisfactory and toler- 

 ably complete explanation of the facts. In order to 

 render the whole subject as clear as possible, it may be 

 advisable not only to recapitulate the most salient 

 features in a concluding chapter, but to deal with a few 

 facts bearing on this new Law of Dispersal that could 

 not well have been introduced into the general subject 

 matter of the volume. 



In dealing with the Migration and Dispersal of British 

 Birds, we found it impossible to make any progress until 

 we had traced out the past geographical and climatic 

 changes, not only in the British Area itself, but in more 

 or less adjacent areas, during late Pliocene and through- 

 out Pleistocene time. We had first to ascertain as 

 correctly as the state of present knowledge admits, the 

 condition and the physical aspects, not only of Europe, 

 but of a great part of Africa, during those periods. 

 Taking the present distribution of species as a guide, we 

 ascertained that a great many of those changes of climate 

 and of geographical conditions upon which astronomers, 

 phj'sicists, and geologists insist, were in absolute harmony 



