Vol. X 

 iS93 



Allen, Origin and Distribution of N. A. Birds. IOI 



these a comparatively low temperature had doubtless become 

 congenial, and from them may have been derived the distinctively 

 arctic and subarctic birds of the present age. They belong 

 mainly to our present circumboreal genera, and are unrestricted 

 by climatic conditions in their dispersal throughout the arctic 

 and subarctic regions. 



Other forms proved less flexible, and remained in latitudes 

 more nearly corresponding to the climatic conditions of pre- 

 glacial times. They had, however, before the beginning of the 

 Glacial Period, become broadly dispersed, and now are found in 

 widely disconnected areas. We have thus a reasonable explana- 

 tion of the disconnected distribution of congeneric species in such 

 groups as the Tree Ducks, Egrets and Herons, Spoonbills, 

 Flamingoes, Snakebirds, Gannets, Gallinules, Barn and Pigmy 

 Owls, Kites (genus Elamis), Trogons, Barbets, Kingfishers, 

 Swifts, Goatsuckers, Piculets, and a few Passerine birds. On 

 the other hand, doubtless many of the peculiar tropical types of 

 land birds were already restricted to somewhat near their present 

 limits, and that they have never had a much wider dispersion 

 than they have at the present day. Many of them are also pos- 

 sibly of comparatively modern origin. It is only on this suppo- 

 sition that we can account for the numerous peculiar types that at 

 present characterize the subtropical and tropical areas of the 

 several continents. 



It is not probable, for example, that such exclusively trop- 

 ical American families as the Formicariidas, the Dendrocolap- 

 tidae, the Galbulida?, the Todidas, the Toucans, the Motmots, 

 the Cotingas, etc., have ever had a much wider range than 

 now. It seems also probable that such distinctively Amer- 

 ican types as the Hummingbirds, the Icteridag, the Tyrant Fly- 

 catchers, the Tanagers, the Vireo? and the Mniotiltidas, which 

 for the most part have their centers of abundance in the tropics, 

 with merely outlying members in temperate North America, have 

 never had a wider range than at present, and that most of their 

 outlying genera and species have, under the intense struggle for 

 existence in the overcrowded tropics, become gradually somewhat 

 modified to adapt them to slightly more boreal conditions, thus 

 in course of time more or less extending the general habitat of 

 the families to which they respectively belong. At the far north 

 they are still cut off from further extension by an insuperable 



