140 The Jj'ish Naturalist. April, 



happened upon an island. For example, in parts of Africa 

 and of South America— to judge from what has been told us 

 by Livingstone, Darwin, and other writers — almost the whole 

 mammalian fauna has sometimes been exterminated by 

 drought ; and that extermination, however wholesale, is still 

 only temporary, because, w 7 hen the drought is over, animals 

 from the neighbouring parts of the continent, chiefly of the 

 same kinds, come in to take the places of those that have died. 

 On an island, there can be no such renewal ; a species once 

 exterminated, unless it has means of transportal by sea, is ex- 

 terminated for ever. " Some sixty years ago," says Mr. 

 Distant 1 , "the Sousliks (Spcmiophihts sp.) suddenly disap- 

 peared iu the neigbourhood of Sarepta, in south-eastern 

 Russia, in consequence of some epidemic ; and for years no 

 Sousliks were seen in that neighbourhood." The Sousliks, it 

 seems, are now as numerous about Sarepta as they formerly 

 were ; but that is probably due to the fact that the epidemic 

 which destroyed them did not prevail over the whole of south- 

 eastern Russia. A similar visitation on an island would have 

 killed them beyond recall — unless the island was large enough, 

 or varied enough in its features, to possess within itself an 

 area which the epidemic did not affect. 



This may explain the absence of some British animals from 

 Ireland ; because Ireland is not only smaller, but has also a 

 more uniform character, than the island of Great Britain. 

 The Squirrel is believed by Mr. Harvie-Brown to have become 

 extinct in the north of Scotland towards the close of the 

 eighteenth century; but it returned during the latter part of 

 the nineteenth, as an immigrant from more southern parts, 

 The same thing might have occurred in Ireland, but here the 

 uniformity of conditions would lessen the chance of the 

 animal's being able to return. I think, however, that the 

 poverty of island faunas where the islands are at all large re- 

 quires some further explanation than this ; and from a con- 

 siderable number of isolated facts, none of them very strong 

 in themselves, but perhaps worth some closer investigation, 

 I have come to suspect that species are sometimes limited 

 in their range by diminished fertility towards its outermost 



1 Zoologist, September, 1905. 



