Vol.X 



"' x "| Allen, Origin and Distribution of N. A. Birds. 



99 



since buried in the Tertiary deposits of both North America and 

 Eurasia. Furthermore, there is abundant evidence of a consider- 

 able interchange of life between the two northern continents at a 

 time when there was no climatic barrier, as now, to the north- 

 ward extension of subtropic types ; and that Eurasia was in part 

 populated by types originating in North America ; and also that 

 North America has derived a portion of its life from the Old 

 World. There is no reason to suppose that birds were then any 

 less restricted in their means of dispersion than now, in compari- 

 son, for example, with mammals, insects, land mollusks and 

 plants. 



The past history of birds, while so defective in comparison with 

 that of some other groups, affords proof of the former much wider 

 dispersion of certain types than obtains at the present time. 

 While for this class the geological record is so imperfect, it 

 suffices to show that what are now strictly tropical genera, for 

 example, formerly reached southern and central Europe. It also 

 affords evidence that birds in Tertiary times were not so very 

 different from the birds of to-day- The leading genera of the 

 raptorial, gallinaceous, wading and swimming birds were much 

 the same as now 1 ; with them existed other genera which have 

 since become extinct, while many of the now prevalent forms 

 were absent, and have doubtless come later into existence. The 



1 Among the existing genera 

 America are the following, most 

 Eocene. 



of birds found in the Tertiary of Europe and North 

 of which date back to the Miocene, and some to the 



