780 On the Avifauna of Central America. 



The dense forests of Central Americaj which at the 

 southern edge of the great Mexican plateau sweep north- 

 ward on either side along the seaboard to Mazatlan on the 

 west and Matamoras on the east^ have exercised a profound 

 selective influence on the avifauna of this region, as well as 

 on Mexico. The characteristic gloom and barrenness of the 

 interior of such forests must have proved a formidable 

 barrier to the northward advance of ground-dwelling 

 species. Certain of the Tinaraous, however, which have 

 become adapted to the conditions of forest-life, have con- 

 trived to penetrate as far as Mexico. The Rheas, however, 

 beinij, like most of the Tinamous, birds of the open country, 

 have been excluded. The real life of such forests is con- 

 centrated upon the tops of the trees, which teem with life. 

 Here is a blaze of colour and an abundance of fruit and 

 insect-life ; and hence may be explained the presence in 

 Mexico of Macaws, Penelopes, Guans, and Curassows. 

 But the presence of these birds makes the absence of so 

 many other tropical South American species the more 

 conspicuous. 



Finally, it is to be observed that though the Isthmus of 

 Panama forms a bridge connecting the South American 

 Continent with the smaller tropical land-area of Central 

 America, this bridge has formed by no means an open high- 

 way for the intermingling of the faunas of north and south. 

 On the contrary, only such species as were adapted to a 

 tropical temperature and the conditions of a forest-life 

 could travel along the lowland route, while such only as 

 were adapted to a low temperature and the conditions of 

 the terrain could pass southward over the highland route. 

 This limited interchange of species between the two areas 

 has only been effected by a process of slow infiltration, not 

 by a rapid interchange. In the Isthmus of Panama, then, 

 we have, perhaps, the best actual demonstration of the limi- 

 tations and extent of a land-bridge as a highway between 

 otherwise isolated areas that the world can afford. 



