232 Trotter, Land-Bird Fauna of N. E. America. [july 



and our Vireos and Tanagers; many of our Sparrows and Gros- 

 beaks; all of our Gnatcatchers, and the Mockingbirds, some of 

 our Wrens, and a few of our more southern genera of Warblers, 

 as the Yellowthroats and Redstarts," are clearly of tropical origin. 

 Probably this influx of plateau and tropical types into the eastern 

 region was a very slow and gradual movement which took place 

 during and after the addition of the marginal Tertiary seafloor to 

 the southeastern portion of the continent, which increased the land 

 area to the extent of the present southern and Atlantic coastal 

 plain. Much of Cuba, the peninsula of Yucatan, and the eastern 

 seaboard of Mexico was uplifted about this time. A third element 

 appears to have had its origin within the limits of the region itself, 

 though many of the genera are represented by numerous species 

 in the western portion of the continent. Of the more strictly 

 eastern genera may be mentioned Dolichonyx, Mniotilta, Pro- 

 tonotaria, Helinaia, Helmitherus, Helminthophila (the larger 

 number of species), Dendroica (mainly eastern), Siurus, Oporornis, 

 Sylvania, Galeoscoptes, Cistothorus, and Telmatodytes. The fourth 

 element in our bird fauna is the Old World boreal group of genera 

 already mentioned and a number of species of pronounced Old 

 World affinities, as the Robin, the Hylocichline Thrushes, the Tit- 

 lark, the Barn, Cliff, and Bank Swallows, the Shrikes, Crows, and 

 Shore Larks, which have been more or less modified from Pala?- 

 arctic types. 



It is next to impossible to say in which portion of the continent 

 many of the purely indigenous or autochthonous forms had their 

 origin, for they are spread across the land from ocean to ocean in a 

 succession of closely allied species or as local races. This is espe- 

 cially true of most of our genera of indigenous sparrows. Un- 

 doubtedly there occurred sometime during middle and late Tertiary 

 times an extension of plateau types into the humid Eastern Prov- 

 ince, and, conversely, a spread of eastern forms into the arid 

 districts of the Plateau region, while at the same time an influx of 

 tropical forms made their appearance, coming probably in the 

 main from a tropical land area in the southwest and following the 

 widening Gulf margin of the continent. How far north this 

 preglacial bird fauna of diverse origin may have spread it is im- 

 possible to say, but some forms, even of tropical origin, undoubtedly 



