OF MAMMALIA. 6ii 



evidence that these ' ' Continental " islands have been only 

 recently separated from the mainland, and a sufficient time 

 has not elapsed for the mammalian fauna to diverge from 

 the parent stock. In the case of Britain, for example, it is 

 generally accepted that in Pleistocene times the North Sea 

 was dry land, thus accounting for the identity of fauna at 

 that time between Britain and the Continent. The extinc- 

 tion in Britain of many continental types has not yet been 

 explained, though of course the wolf, beaver, wild boar and 

 brown bear have been exterminated by man, by whose agency 

 have also been introduced the rabbit, brown and black rats 

 and fallow deer. 



In fact, the faunistic character of an island or a continent, 

 like the structure of an organism, is a complex relationship 

 in space, the facts of which are easily attainable by observa- 

 tion. The explanation of the facts in each case is obscure, 

 depending upon the relationship in time, a factor in which 

 the investigating unit is too severely limited to permit of 

 anything beyond the slowest progress. 



