CHAP. I1.] DISPERSAL AND MIGRATION. 13 
many animals are restricted to the slopes of the Himalayas 
or to the mountains of Central India, the flat 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 Rio 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 Ateles 
paniscus extends to the north bank of the river while Lagothrix 
humboldtiz 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, Rio 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 
