178 ■ Field and Forest Rambles. 



we shall have to refer to that of Greenland on the north-east, 

 and to the imperfect data in connection with the birds of 

 north-eastern Asia on the one side, and South America on 

 the other. Thus it is apparent that, as regards North 

 America, we are at present only enabled to determine with 

 approximate precision the laws which, in the case of the 

 birds of Europe, have long been clearly defined. It is a fact 

 generally accepted by naturalists that about 60 species of 

 birds are common to the northern continents of the New and 

 Old Worlds ; thus, by eliminatingthese we find there are about 

 444 species peculiar to Europe, and no less than 680 strictly 

 North American birds. As regards the other continents above- 

 mentioned, it is unnecessary, for the illustration of our subject, 

 to draw comparisons. It is apparent, therefore, as far as 

 North America and Europe are concerned, that their birds 

 differ considerably; and indeed the same may be said of their 

 natural objects generally. 



With reference to the geographical distribution and migra- 

 tions of the birds of North America, it was found by Professor 

 Baird* that the general principles in regard to these points 

 were demonstrable from a consideration of the climate and 

 physical characters of the continent; accordingly, he divides 

 North America into two grand ornithological (nay, zoological 

 as regards the class vertebrata) regions, — viz., the eastern, or 

 Atlantic ; and the western, or Pacific. Referring to the 

 accompanying sketch map, the eastern division extends from 

 the Atlantic seaboard, westward across the Alleghanies and 

 over the valley of the Mississippi and its fertile prairies, to 

 about the ioo° of longitude, or to the beginning of the sterile 

 plains. 



The western division begins at the western border of the 



eastern, or along the sterile plains of the trans-Mississippi 



country, and extends across to the Pacific Ocean. These 



are better defined, on the chart where the part shaded repre- 



* "American Journal of Science," vol. xli. 



