184 ABSENCE OF TERRESTEIAL [Chap. XIH 



by the stupendous degradation which they have sufEered, 

 and by their tertiary strata: there has 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 Archipela- 

 goes, 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 is- 

 lands? On my view this question can easily be an- 

 swered; 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 is- 

 lands. 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 mam- 

 mals. 



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 



