GEOGRAPHICAL DISTRIBUTION 411 



of discontinuous distribution is, for the moment, our immediate 

 concern. 



One of the best examples of discontinuous distribution is 

 afforded by the order of Pouched Mammals (Marsupialia), now 

 mainly limited to the Australian region, though also represented 

 in America by the Opossums and one other form (Ccenolestes). 

 Without the aid of the geological record the reason for this would 

 ever remain a matter of the merest conjecture. We know, how- 

 ever, from the evidence this record affords, that in the remote 

 past Pouched Mammals were common enough in Europe, and 

 there are enough facts upon which to base the view that the 

 earliest representatives of the order were evolved in the land- 

 mass of Eurasia. From this area the Pouched Mammals 

 gradually spread, entering what are now America and Australia 

 over tracts of land since submerged beneath the sea. Elsewhere, 

 owing to the competition of more highly specialized mammals, 

 they have died out. But the Australian region having been cut 

 off from the northern land-mass before the higher mammals had 

 a chance of entering it, the pouched forms of that region had a 

 field free from serious competition, in which have since been 

 evolved numerous species adapted to many diverse modes of life. 

 In America they had a harder struggle for existence, and at the 

 present time are poorly represented there, chiefly by Opossums, 

 the ancestors of which no doubt reached the New World by one 

 or more formerly existing land-bridges in the north. There is 

 also good reason for thinking that South America also received 

 a population of pouched mammals from Australia, by means of 

 a southern land -bridge, of which some existing islands appear 

 to be remnants. This Australian stock has since died out almost 

 entirely, being now only represented by two small species of 

 Opossum- Rats (Ccenolestes), native to Colombia and Ecuador. 



Discontinuous distribution explained on somewhat different 

 lines is exhibited by the Lung- Fishes (Dipnoi), now represented 

 only in the fresh waters of Africa, South America, and Queens- 

 land (see vol. i, p. 264). Once more a clue is afforded by the 

 geological record, from which we know that the ancestors of the 

 Lung- Fishes were at one time a dominant and widely distributed 

 marine group. Hard pressed by fishes better adapted to life in 

 the sea, some of them took refuge in estuaries, ultimately pass- 

 ing into the fresh waters of the land. It is only these which 



