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Ch. XXXVIII.] MAMMALIA OF THE ETHIOPIAN EEGIOX. 



343 



tlie Great Desert was submerged beneatli the sea in the 

 Pliocene Period ; so that assuming that species haye onlj 

 one birthplace^ we can account for their distinctness in 



these two regions^ which were separated first hj a barrier of 



water and afterwards by one of sand. 



The geographical distribution of reptiles agrees as a 

 general rule with that of the mammalia and birds ; but a 

 discrepancy has been pointed out in the Pala}arctic region. 

 Although the batrachians of Japan are all Palsoarctic^ the 

 snakes agree in genera and species with those of the more 

 southern parts of Asia or the Indian 



region. 



which we 



suggests 



: he 



shall have presently to consider. Mr. Wallace 

 the following explanation of this apparent anomaly 

 reminds us that Dr. Giinther has shown that snakes are a 

 preeminently tropical group, decreasing rapidly in the tem- 

 perate regions, and absolutely ceasing at 62° IST., wdiereas 

 the batrachians are almost as largely developed in northern 

 as in trox3ical latitudes, being able to support, partly by aid 

 of hybernation, a very cold climate. We may therefore 



ISTorther 



almost 



of islands 5 in which case it might have received its birds 

 mammals, and batrachians from 



the Pala^arctic resfion. 



whereas it could have derived but few or no snakes from the 

 same quarter, since the great cold extends to a much lower 

 latitude in Eastern Asia than in Western Euroj^e. If at a 

 subsequent period Japan became connected with Southern 

 Asia through the Loo-choo and Majicosima islands, it mio-ht 

 then have been colonised by snakes of Indian origin, whick 

 would easily establish themselves in a region unoccupied by 

 any representatives of the same class. Batrachians, on the 

 contrary, as well as the birds and mammals of Southern Asia, 

 would find a firmly established Palsearctic population ready 

 to resist the invasion of all intruders.* 



Ethiopian recjion.~The next or fourth zoological province 

 is the Ethiopian, including Africa south of the Great Desert, 



part of Africa 



Madag 



That 



this 



* Wallace on Zoological and Botanical Geography, Nat. Hist. Eev. 1864, p. 114. 



