VOl i90? VI ] Trotter, Land-Bird Fauna of N. E. America. 233 



reached a high latitude, very probably during the warm Miocene 

 and early Pliocene times, and their descendants may possibly now 

 be represented by those migrants which breed far within the limits 

 of the Boreal Zone. During Miocene times there were extensive 

 land connections between Asia and Northwest America and very 

 likely a much closer land relation between Europe and America. 

 It was during this time, no doubt, that the influx of our Palaearctic 

 types occurred, and it is a significant fact that all of these genera are 

 of extensive range and of markedly northern distribution, such for 

 example as the Passerine genera Merula, Regulus, Parus, Sitta, Cer- 

 thia, Anthus, Hirundo, Petrochelidon, Riparia, Ampe/is, Lanius, 

 Pinicola, Carpodacus, Loxia, Acanthis, Passerina, CcUcarius, 

 Corvus, and Otocoris. 



Throughout an immense lapse of time, time that must be reck- 

 oned in hundreds of thousands of years, during which the great 

 Keewatin and Laurentide glaciers pushed their ice sheets beyond 

 the present site of the Great Lakes and the Mohawk Valley, forcing 

 southward the animal and plant life into an area of high biotic 

 tension, a widespread change in types must have taken place. The 

 more primitive forms have undoubtedly disappeared. Only 

 occasionally may we pick up a trace of this ancestry in some fleeting 

 juvenal phase of plumage. Modifications of type went on; differ- 

 entiation into new genera, species, and varieties through molecular 

 changes in pigmentation, in size and shape of bill and feet, of wings 

 and tail, and in the deep-seated structure of the germ plasm. 

 Diversity of structure went hand in hand with diversity of habit 

 and of habitat. It was a period of profound environmental mould- 

 ing, intensified by the effect of the glaciers on the land and its life. 

 From our limited point of view the array of species and varieties 

 which we see to-day seem peculiarly stable in their features and their 

 adaptations. But the dynamic influences of environment are 

 ceaseless if inconspicuous. Species and faunas alike are but 

 passing phases in the vast cosmic processes of a continent's history. 



