V i°893 X ] Allen, Origin and Distribution of N. A. Birds. IK 



Cold Temperate 120, and in the Warm Temperate 205. 

 (2) The decrease of Old World forms from the north southward, 

 in the Arctic belt 91 per cent, of the genera being circumpolar, 

 82 per cent, in the Cold Temperate, and 37 to 23 per cent, in 

 the Warm Temperate, according to whether the northern or 

 the southern edge of the belt be considered. (3) As the dis- 

 tinctively American genera increase in passing southward, they 

 became more or less differentiated into eastern and western types. 

 Taking land birds alone, to the exclusion of pelagic and strictly 

 maritime, the number of peculiarly eastern genera increases from 

 two or three in the Cold Temperate to 17 in the Warm 

 Temperate, and in the western from three or four in the Cold 

 Temperate to 26 in the Warm Temperate, exclusive in each case 

 of intrusive, essentially tropical forms which if taken into 

 account would still further differentiate the two regions. Besides, 

 as is well known, many genera which range across the continent 

 are represented by different species on the two sides, while most of 

 the continental species are differentiated into eastern and western 

 subspecies, and frequently into a considerable number of local 

 races. 



We may now inquire, Whence was derived the present avifauna 

 of North America? It is evident from the facts already presented 

 that it is made up of four prominent elements. The first con- 

 sists of types common to a large part of the northern hemi- 

 sphere, which more and more prevail as we proceed from the 

 south northward, till in subarctic and arctic America we meet with 

 little else. The second consists of a rather strong infusion at the 

 southward of types of almost universal distribution over the 

 warmer temperate and intertropical latitudes. Third, a very 

 prominent tropical American element, developed to the maxi- 

 mum just along our southern border and fading out gradually to 

 the northward, little being left of it after we enter the Cold 

 Temperate. Fourth, a very conspicuous autocthonous ele- 

 ment, reaching its maximum in the Warm Temperate, and 

 continuing prominent far into the Cold Temperate. This is 

 made up partly of strongly modified Old World types, but 

 mainly of peculiar genera, or at least peculiar species, derived 

 obviously from tropical American stock, which entered our 

 borders partly by way of the West Indies, but mainly by way of 

 Central America and Mexico. 



