184 ABSENCE OF TERRESTRIAL [Chap. XIII. 



ancient, as shown by the stupendous degradation which 

 they have suffered, and by their tertiary strata : there 

 has also been time for tlie production of endemic species 

 belontringr 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 tliis 

 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 main- 

 land. 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 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 



