Avifauna of Central America. 775 



iu the one case by an Afro-Brazilian bridge, long since sub- 

 merged, and in the other by tlie still-existing Isthmus of 

 Panama, which has persisted without interruption since 

 Miocene times. 



While there is evidence to show that in some cases 

 northern species have succeeded in extending their rauge, 

 not only into subtropical and tropical regions, but into the 

 temperate zone beyond, as in the case of the Troglodytidae, 

 the southern tropical types do not display similar powers of 

 adaptation. Species evolved within the temperate regions 

 rarely succeed in establishing a hold within the tropical 

 zone, save when they are able to seize upon elevated plateaux 

 or mountain-ranges. This being so, it is clear that the 

 character of the avifauna of the areas now to be discussed 

 is not merely determined by " land-bridges," but also by 

 matters of climate and temperature, and especially the latter. 

 With a low mean temperature, which is as much a matter of 

 altitude as of latitude, Central America would have been as 

 fi'ee from invasion from the south as it would have been if 

 the Isthmus of Panama had remained submerged. 



The effectiveness of aquatic barriers being admitted, then 

 the existence of the Afro-Brazilian land-bridge becomes 

 almost a necessity to account for the presence, in South and 

 Central America, of types indubitably of African afiinities. 

 Except by means of a continuous land-surface, and an equally 

 continuous environment — that is to say, of tropical condi- 

 tions, — how are we to account for the distribution of the 

 Fiufoots, for example, which extend across central Africa 

 and eastwards into Burma, Assam, and Sumatra on the one 

 hand, and into Central and South America on the other. 

 There is not the slightest ground for supposing that these 

 birds were ever migratory, in the usual sense of the term. 

 The migratory instinct, indeed, is always associated with 

 strongly contrasted seasonal changes, such as are wanting 

 in tropical and sub-tropical regions. What applies in 

 the case of the Finfoots applies with equal force to the 

 Peristeropod Galliformes, the Capitonidee, and the Trachaeo- < 

 phone and Oligomyodian Passeres, If the Afro-Brazilian 



