162 Trotter, Relation of Genera to Faunal Areas. [aptIi 



the Wood Thrush, H. mustelina, has not advanced beyond the 

 Transition Zone. 



The Genus Dendroica of the Mniotiltidse presents the antithesis 

 of this close hkeness of specific forms. We have here a group of 

 thirty-four species that are widely different from one another in 

 color pattern and with breeding ranges, in many cases, coterminous, 

 or at least markedly overlapping. A geographical analysis of the 

 genus shows that the species fall into two equal numerical groups. 

 One group of seventeen well-defined forms has a strictly northern 

 breeding range as compared with another group of seventeen 

 species that is mainly extra-limital, breeding in the sub-tropical 

 or the tropical domain, certain forms being confined to insular 

 areas in the Caribbean. Four species of this second group have 

 forms that breed in the North American region, namely D. cestiva, 

 D. auduhoni, D. gracice, and D. vigorsi. Of the first group of 

 seventeen strictly North American species only three are limited 

 to the western side of the continent — D. uigrescens, D. toimi- 

 sendi, and D. oecidentalis, the remaining fourteen being highly 

 characteristic of the eastern fauna. 



Taking into account the large numerical element in this genus 

 and the great variety displayed by its forms, together with the fact 

 that one half of the recognized species are still confined to the 

 tropical or sub-tropical area, there seems some evidence for be- 

 lieving that this group of birds is of considerable antiquity and that 

 its area of characterization was somewhere in the Middle American 

 region of tropical environment, possibly at a time when the Tertiary 

 land borders of the Gulf and Caribbean were much more extensive 

 than at present and when certain of the now island masses were 

 more closely connected with the main continental land. With the 

 disappearance of glaciers from the northern region certain primitive 

 types spread northward, and I think we may recognize the van- 

 guard of this movement in such species as D. striata, D. castanea, 

 D. palmarum, D. townsendi, D. magnolia, D. tigrina, D. cestiva 

 and D. coronata. All of these forms reach a high northern latitude 

 in the breeding season and some like the Black-poll and Palm 

 Warblers, the Myrtle and Magnolia Warblers have been lured far 

 into the Northwest, indeed quite to the Sub-Arctic, by the great 

 stretch of coniferous forest. The wide overlap observed in the 



