Avifauna of Central America. 779 



species have arisen by isolation, each mountain has become 

 an island, cut off from all intercourse with its neighbours. 

 These mountain-tops are as much islands as they would be 

 if by subsidence they were sunk beneath the sea, leaving 

 only their tops exposed. The isolating factor is probably 

 the simple restraint of contentment. The birds do not 

 increase beyond the limits of their food-supply, and suffer 

 no extremes of climate ; hence they have no incentive to 

 travel. The fact that so many distinct species have been 

 evolved here points to a far distant past, when the 

 ancestral stock was generally distributed along the lower 

 ground. 



A precisely similar process of specific differentiation is 

 exhibited in the case of the genus Sporophila, which has 

 spread from Mexico, through Central America, and over the 

 tropical belt by means of this temperature-bridge into 

 South America, where, travelling along the Andes, it 

 has given rise to new species in Peru and Argentina. 

 Similarly, the genus Spinus {Chrysomitris) has contrived to 

 run the whole length of both Americas, from Labrador 

 to Tierra del Fuego and the Falklands — to say nothing of 

 its range outside the New World. 



Instances of this kind could easily be multiplied, but 

 enough has been said to demonstrate the influence of the 

 Mexican and Central American highlands on the avifauna 

 of South America, and to bring out the singularly interesting 

 fact that with an identical highway South America has con- 

 tributed nothing to the avifauna of the northern portion of 

 the continent. 



It has already been remarked that species attuned to a 

 temperate climate rarely develop the capacity to live within 

 the tropics. There are some exceptions to this rule. One 

 such is afforded by the American Wrens, which range from 

 Alaska to Patagonia, inhabiting every possible kind of 

 country. The Brown House- W^ren [Troglodtjtes furvus) of 

 South America, for example, has been described as ubiqui- 

 tous, being equally at home either in the tropical forests, 

 deserts, reed -beds, or the cold uplands of Patagonia. 



