Home Territories 29 
as thoroughfares to bring lowland species far into the otherwise 
mountainous hinterlands. The ultimate limiting factor in such 
cases is probably temperature; wherever the bed of a river rises 
so high above sea-level that night temperatures fall below some 
critical point, the tropical fauna will be excluded. Conversely, 
mountain chains projecting from extensive upland areas will 
often permit faunas characteristic of elevated plateaus to extend 
along their crests far into regions otherwise distinct. 
