38 THE MIGRATION OF BRITISH BIRDS 



availed themselves of such a retreat to the tropics and 

 survived amongst such congenial surroundings down to 

 the present day, yet, so far as I can ascertain, there is 

 not a single trace throughout the Ethiopian region of 

 any such Pleistocene exodus, for the Law of Southern 

 Dispersal forbids it. The evidence also points to the 

 conclusion that the same Law forbade the retreat of 

 these Euro-Asiatic mammals into Africa by way of the 

 Nile. The presence in tropical Africa of the hippo- 

 potamus, the sole surviving form, undoubtedly proves 

 that the range of this species during the Glacial Epoch 

 was Ethiopian. Precisely the same remarks apply to 

 birds. We have abundant evidence of the former exist- 

 ence in Europe during Tertiary time of man)- Oriental 

 and Ethiopian types, such as Collocalia, Trogon, Psittacus, 

 Nccroniis, Liinnatornis, Ccntropus, etc.,^ — all species that 

 had spread northwards and eastwards from southern 

 or equatorial bases and become sedentary in the warm 

 equable climate of Euro- Asia until exterminated by the 

 advent of adverse conditions of life, primarily due to a 

 change of climate ; and that now continue to be solely 

 represented by types that have preserved their identity 



^ Every species that did not BREED south of glacial limits must 

 have perished ; numbers of other species that were resident in 

 many parts of Euro-Asia must also have been exterminated by the 

 adverse change in the climate. Among these were such types as 

 are here instanced. If the advent of a glacial climate could 

 initiate a migratory southern movement or an extension of perma- 

 nent habitat in the same direction, then numbers of such types 

 would have survived and left traces of their southern exodus 

 behind them ; but adverse climatic conditions can never lead to 

 increase or extension of winter or non-breeding area, and extermina- 

 tion is the inevitable result, unless modification may in some cases 

 preserve them from utter obliteration. 



