32 THE MIGRATION OF BRITISH BIRDS 



extends to Palestine, amongst the most notable forms 

 being species of Cossypha and Droinohea. We must not 

 view these forms as remnants left behind in the 

 north when the great and entirely legendary exodus 

 into Africa was in progress, but either as Inter-hemi- 

 sphere {coiif. p. 60) species that have spread from an 

 Antarctic, or Southern Hemisphere, centre of dispersal 

 northwards across the entire Ethiopian region, or from 

 an equatorial base. In Palestine, it may be remarked, 

 we also find a commingling of types peculiar to the 

 Oriental and the FLthiopian regions in such genera as 

 Ncctariiiia and Pycnoiiotus. Such facts demonstrate very 

 clearly that where the land surface is continuous and 

 sufficiently fertile to support life, a commingling of types 

 peculiar to different faunal regions invariably occurs, but 

 in the Northern Hemisphere the invasion is never of a 

 purely Southern character, the encroaching species con- 

 forming entirely to the Law that forbids a Southern 

 extension of area {conf. p, 60), and also confirm the view 

 previously expressed that the Sahara, either as an inland 

 sea or a sterile desert, checked all such mixture of Pahe- 

 arctic and P^thiopian t}'pes in the west of Africa, the 

 division between the two faunas, so far as we know in 

 that district, being a very abrupt one, whilst in the cast 

 of Africa, where no such barrier exists, the faunas of 

 the two regions blend in a very suggestive manner. It 

 is a most remarkable fact that the fauna and flora of 

 P2urope and North-west Africa have retained their 

 homogeneity although separated by one of the deepest, 

 widest, and most ancient seas in the world — phenomena 

 that suggest a long-enduring and-recent land connection, 

 which we know existed at no remote epoch, as the 



