THEORIES OF MIGRATION 35 



tern its extent is governed by the reach of the 

 daggerlike bill that threatens all who crowd within 

 reach. The cardinal, the mocking-bird, the thrush, 

 meadowlark, or vireo, on the other hand, controls a 

 larger area, a tree or a group of trees, or a stretch of 

 thicket or grassland. Intruders of the same species 

 may come near, but may not tarry within the limits 

 of this tract without doing battle with the one who 

 has preempted it. 



Let us now turn back some hundreds of thousands 

 of years, to a period in the Tertiary near the close of 

 the Pliocene. Geologists inform us that during the 

 Miocene and Pliocene, when the genera to which our 

 modern birds belong were attaining their develop- 

 ment, in North America and elsewhere in the north- 

 ern hemisphere climatic conditions, though perhaps 

 not especially warm, were more or less equable from 

 the Equator to within the boundary of the Arctic 

 Circle. Zonal bands increasing regularly in warmth 

 to the southward were not as strongly indicated as 

 at the present day, and in those epochs there would 

 not have been necessity for regular migration of the 

 birds then in existence, further than a retreat from 

 the far northern regions of extended night, when the 

 sun was south of the Equator, a seasonal shifting 

 with changing food-supply, or change with expand- 

 ing range among dominant species such as is found 

 in the Tropics to-day, or as is recorded in the conti- 

 nent of Australia. 



