GEOGRAPHICAL DISTRIBUTION 355 



presence of endemic bats on islands, with the absence 

 of all terrestrial mammals. 



Besides the absence of terrestrial mammals in rela- 

 tion to the remoteness of islands from continents, 

 there is also a relation, to a certain extent independent 

 of distance, between the depth of the sea separating 

 an island from the neighbouring mainland, and the 

 presence in both of the same mammiferous species or 

 of allied species in a more or less modified condition. 

 Mr. Windsor Earl has made some striking observations 

 011 this head in regard to the great Malay Archipelago, 

 which is traversed near Celebes by a space of deep 

 ocean ; and this space separates two widely distinct 

 mammalian faunas. On either side the islands are 

 situated on moderately deep submarine banks, and 

 they are inhabited by closely allied or identical quad- 

 rupeds. No doubt some few anomalies occur in this 

 great archipelago, and there is much difficulty in form- 

 ing a judgment in some cases owing to the probable 

 naturalisation of certain mammals through man's 

 agency ; but we shall soon have much light thrown 

 on the natural history of this archipelago by the 

 admirable zeal and researches of Mr. Wallace. I have 

 not as yet had time to follow up this subject in all 

 other quarters of the world ; but as far as I have gone, 

 the relation generally holds good. We see Britain 

 separated by a shallow channel from Europe, and the 

 mammals are the same on both sides ; we meet with 

 analogous facts on many islands separated by similar 

 channels from Australia. The West Indian Islands 

 stand on a deeply submerged bank, nearly 1000 fathoms 

 in depth, and here we find American forms, but the 

 species and even the genera are distinct. As the 

 amount of modification in all cases depends to a certain 

 degree on the lapse of time, and as during changes 

 of level it is obvious that islands separated by shallow 

 channels are more likely to have been continuously 

 united within a recent period to the mainland than 

 islands separated by deeper channels, we can under- 

 stand the frequent relation between the depth of the 



