330 ABSENCE OF TERRESTRIAL [CHAP. XIII. 



also been time for the production of endemic species belonging to 

 other classes ; and on continents it is known that new species of 

 mammals appear and disappear at a quicker rate than other and 

 lower animals. Although terrestrial mammals do not occur on 

 oceanic 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 Mauritius, 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 transported 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 on far distant islands. 

 Hence we have only to suppose that such wandering 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 terrestrial 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 mammalian inhabi- 

 tants. Mr. Windsor Earl has made some striking 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 mammalian faunas. On either side the 

 islands stand on a moderately shallow submarine bank, and these 

 islands are inhabited 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 we find American 

 forms, but the species and even the genera are quite distinct. As 

 the amount of modification which animals of all kinds undergo 

 partly depends on the lapse of time, and as the islands which are 

 separated from each other or from the mainland by shallow 

 channels, are more likely to have been continuously united within 

 a recent period than the islands separated by deeper channels, we 

 can understand how it is that a relation exists between the depth 



