164 Trotteb., Relation of Genera to Faunal Areas. LApril 



thousand years. It is probable that many existing species of 

 birds frequented these ancient meadows and the encroaching 

 woodlands of birch, alder and conifer. If not the identical species 

 then at least a very near and closely similar parent form that had 

 split off from a still more remote parental stock. 



I have taken the genus Hylocichla and the genus Dendroica as 

 presenting very wide contrasts in the number of specific forms and 

 in degree of likeness. Any genus of land birds that we will analyze 

 must reveal some features that point to its history in connection 

 with the origin and distribution of its several species and their 

 varieties. If a genus stands for anything it must stand for this 

 origin of its several species from a common ancestral type, near or 

 remote. If the distribution of a genus means anything it means 

 the history of its species in relation to changes through environ- 

 ment and the fixation of characters by segregation. Overlap or 

 coterminous breeding range is the logical sequence of the spread 

 of a species or variety into the territory of another, either after the 

 one in advance has become fixed or in habitat differentiation if 

 advancing at the same time. The Transition Zone Fauna is a 

 wide expression of this overlap and is a clear indication of the 

 advance of types toward the north. It is not, however, a perma- 

 nent expression, nor is any faunal area permanent. Viewed in the 

 immense lapse of time it is a momentary glimpse of an endless 

 biograph. I am constantly interested in reading items in the 

 pages of ' The Auk ' that refer to some species of bird observed in a 

 locality to the north of its general breeding range. 



In conclusion let me sum up the somewhat rambling matter of 

 this paper in the following brief statements : — 



(1) — The genus means an ancestral type that has split up 

 into its present component of species and their varieties under the 

 influences of geographic and habitat environment. 



(2) — The forms thus divided become fixed through segregation 

 in the breeding area either through difference of habitat or by 

 extension of range. 



(3) — Each original type must have possessed, and its descen- 

 dants probably still possess, a certain greater or less resistance to 

 disruption. A genus represented by a single species, or at most 

 by a very small number, would seem to indicate a high degree of 

 resistance, even though spread over a widely varied territory. 



