40 THE MIGRATION OF BRITISH BIRDS 



Europe during the last glacial period. After the range 

 of these animals had been contracted into Africa by 

 extermination down the land passages, if you will, which 

 existed between that continent and Europe, we can 

 readily understand how a portion of these species might 

 have been left isolated in North-west Africa after the 

 submergence of such passages, and consequently never 

 re-appeared by Post-Glacial Emigration in Europe with 

 the return of milder climatal conditions. Here, sub- 

 jected to comparatively easy conditions of life, for 

 abundance of food could be obtained, many of them 

 were doubtless able to subsist even to the borders of the 

 Southern Sea or arid desert ; and as this area eventually 

 became studded with oases and scjittercd vegetation, the 

 descendants of the individuals of many of the same 

 species which entered Africa in Miocene or early Pliocene 

 ages gradually spread northwards or westwards, so that 

 the distribution of the species, once discontinuous, eventu- 

 ally became uninterrupted as at present. The Sahara 

 has unquestionably been peopled from the east and 

 south. Emigration commenced from both points as 

 soon as life could be supported there ; from the east b}- 

 species in search of an extension of range, and from the 

 south by species similarly extending their areas of dis- 

 persal northwards in summer, or during the period of 

 reproduction. The presence in this area, however, of 

 endemic forms of these southern and eastern animals, 

 seems to suggest that the colonization of the old Eocene 

 sea-bed in the south was not very extensive, and took 

 place under more or less unfavourable conditions. Even 

 where mammalian and reptilian affinities are not with 

 European species they are with Asiatic forms and not 



