chap, ii.] DISPERSAL AND MIGRATION. 13 



many animals are restricted to the slopes of the Himalayas 

 or to the mountains of Central India, the fiat valley of the 

 Ganges forming a limit to their range. In other cases, however, 

 it is the river rather than the valley which is the barrier. In 

 the great Amazonian plains many species of monkeys, birds, and 

 even insects are found up to the river banks on one side but do 

 not cross to the other. Thus in the lower part of the Eio Negro 

 two monkeys, the Jacchus bicolor and the Brachiurus couxiou, are 

 found on the north bank of the river but never on the south, 

 where a red- whiskered Pithecia is alone found. Higher up Atelcs 

 paniscus extends to the north bank of the river while Lagothrix 

 luimboldtii comes down to the south bank; the former being a 

 native of Guiana, the latter of Ecuador. The range of the birds 

 of the genus Psophia or trumpeters, is also limited by the rivers 

 Amazon, Madeira, Eio Negro and some others ; so that in these 

 cases we are able to define the limits of distribution with an 

 unusual degree of accuracy, and there is little doubt the same 

 barriers also limit a large number of other species. 



Arms of the Sea as Barriers to Mammals. — Very few mammals 

 can swim over any considerable extent of sea, although many can 

 swim well for short distances. The jaguar traverses the widest 

 streams in South America, and the bear and bison cross the 

 Mississippi ; and there can be no doubt that they could swim over 

 equal widths of salt water, and if accidentally carried out to sea 

 might sometimes succeed in reaching islands many miles distant. 

 Contrary to the common notion pigs can swim remarkably well. 

 Sir Charles Lyell tells us in his " Principles of Geology " that 

 during the floods in Scotland in 1829, some pigs only six months 

 old that were carried out to sea, swam five miles and got on 

 shore again. He also states, on the authority of the late Edward 

 Forbes, that a pig jumped overboard to escape from a terrier in 

 the Grecian Archipelago, and swam safely to shore many miles 

 distant. These facts render it probable that wild pigs, from 

 their greater strength and activity, might under favourable cir- 

 cumstances cross arms of the sea twenty or thirty miles wide; 

 and there are facts in the distribution of this tribe of animals 

 which seem to indicate that they have sometimes done so. Deer 



