CHAP. XXXIII.] PHYSICAL GEOGB,APHr. 293 



because many terrestrial mammals and some reptiles have 

 the means of passing over short distances of sea. But in 

 these cases the number of species that have thus migrated 

 will be very small, and there will be great deficiencies 

 even in birds and flying insects, which we should imagine 

 could easily cross over. The island of Timor (as I have 

 already shown in Chapter XIII.) bears this relation to 

 Australia ; for while it contains several birds and insects 

 of Australian forms, no Australian mammal or reptile is 

 found in it, and a great number of the most abundant and 

 characteristic forms of Australian birds and insects are 

 entirely absent. Contrast this with the British Islands, in 

 which a large proportion of the plants, insects, reptiles, 

 and Mammalia of the adjacent parts of the continent are 

 fully represented, while there are no remarkable defi- 

 ciencies of extensive groups, such as always occur when 

 there is reason to believe there has been no such connexion. 

 The case of Sumatra, Borneo, and Java, and the Asiatic 

 continent is equally clear ; many large Mammalia, terres- 

 trial birds, and reptiles being common to all, while a large 

 number more are of closely allied forms. Now, geology 

 has taught us that this representation by allied forms in 

 the same locality implies lapse of time, and we therefore 

 infer that in Great Britain, where almost every species 

 is absolutely identical with those 'on the Continent, the 

 separation has been very recent ; while in Sumatra and 



