ABSENCE OF BATRACHIAXS 437 



islands, aerial mammals do occur on almost every island. 

 New Zealand possesses two bats found nowhere else in the 

 world: Norfolk Island, the Viti Archipelago, the Bonin 

 Islands, the Caroline and Marianne Archipelagoes, and Mau- 

 ritius, all possess their peculiar bats. Why, it may be asked, 

 has the supposed creative force produced bats and no other 

 mammals on remote islands? On my view this question can 

 easily be answered ; for no terrestrial mammal can be trans- 

 ported across a wide space of sea, but bats can fly across. 

 Bats have been seen wandering by day far over the Atlantic 

 Ocean ; and two North American species either regularly or 

 occasionally visit Bermuda, at the distance of 600 miles from 

 the mainland. I hear from Mr. Tomes, who has specially 

 studied this family, that many species have enormous ranges, 

 and are found on continents and en far distant islands. 

 Hence we have only to suppose that such v.andering species 

 have been modified in their new homes in relation to their 

 new position, and we can understand the presence of endemic 

 bats on oceanic islands, with the absence of all other terres- 

 trial mammals. 



Another interesting relation exists, namely between the 

 depth of the sea separating islands from each other or from 

 the nearest continent, and the degree of affinity of their mam- 

 malian inhabitants. Mr. Windsor Earl has made some strik- 

 ing observations on this head, since greatly extended by Mr. 

 Wallace's admirable researches, in regard to the great Malay 

 Archipelago, which is traversed near Celebes by a space of 

 deep ocean, and this separates two widely distinct mam- 

 malian faunas. On either side the islands stand on a mod- 

 erately shallow submarine bank, and these islands are inhab- 

 ited by the same or by closely allied quadrupeds. I have not 

 as yet had time to follow up this subject in all quarters of 

 the world ; but as far as I have gone, the relation holds good. 

 For instance, Britain is separated by a shallow channel from 

 Europe, and the mammals are the same on both sides ; and so 

 it is with all the islands near the shores of Australia. The 

 West Indian Islands, on the other hand, stand on a deeply 

 submerged bank, nearly 1000 fathoms in depth, and here wc 

 find American forms, but the species and even the genera are 

 quite distinct. As the amount of modification which animals 



