PALAEARCTIC ELEMENT IN NEW WORLD AVIFAUNA 431 



p. 279) reminded us that this pattern is also true of mammals and of 

 cold-blooded vertebrates. Lindroth (1957) listed twenty-four 

 species of passeriform birds as occurring in both Eurasia and North 

 America. After deducting the swallows and the waxwing, of uncertain 

 origin, and two species no longer considered conspecific with their 

 overseas counterparts {Pants atricapillus with P. borealis; Sitta 

 canadensis with S. corea, kruperi, villosa, and whiteheadi), we are 

 left with nineteen species in common. Of these, no fewer than 

 sixteen, of nine families, are regarded as of Old World origin and as 

 invaders of the New World, while only three species, of two families, 

 are treated as immigrants from the opposite direction. Two of 

 these species, the Lapland Longspur (Calcarius lapponicus) and the 

 Snow Bunting {Plectrophenax nivalis), are panarctic birds that have 

 developed a few weakly characterized subspecies. The third species, 

 on the contrary, is the highly polytypic wren Troglodytes troglodytes. 

 The A.O.U. Check-list (1957), which probably oversplits them, 

 recognizes no fewer than twelve North American races, while the 

 situation in Eurasia may be judged by Vaurie's recent paper (1955), 

 in which he reduced to eight a total of twenty-one proposed races 

 from eastern Asia alone. The question, of course, is why} Why has 

 this single passeriform species been so obviously successful? Why 

 was it one of the very few that has been able to "swim against the 

 tide," and what caused the "tide" in the first place? Why are the 

 palaearctic-derived siskins, pipits, and thrushes found all through 

 South i\merica in suitable habitats, whereas only a single group 

 of New World passeriform birds, the emberizine finches, has radiated 

 at all in the Old World? Such are the unsolved problems of this 

 corner of zoogeography, and most are likely to remain chiefly 

 intellectual exercises, since the likelihood of securing tangible 

 evidence with which to solve them appears small. 



REFERENCES 



Alexander, W. B., and R. S. R. Fitter. 1955. American land birds in west- 

 ern Europe. British Birds, 48: 1-14. 



Amadon, Dean. 1944. The genera of Corvidae and their relationships. 

 Am. Museum Novitates, No. 1251: 1-21. 



American Ornithologists' Union. 1957. Check-list of North American 

 Birds, 5th edition. Published by the Union. 



Darlington, Philip J., Jr. 1957. Zoogeography. John Wiley & Sons, New 

 York. 



