OF MAMMALIA. 611 
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. 
