Directions of the Migrations of Birds. 149 



none, of the characteristic birds of the middle regions of 

 the continent go as far as the Isthmus of Panama or be- 

 yond ; moreover, that the migratory birds of the Rocky 

 Mountain region go only a comparatively short distance 

 southward into Mexico, few of them even reaching Guate- 

 mala, but preponderating on the west coast." Thus there is 

 a general north and south migration, with little intermix- 

 ture of the birds of the Pacific and Atlantic coast regions; 

 moreover, Baird, as observed in the last chapter, has noted re- 

 markable disproportions in size between specimens of the same 

 species of both mammals and birds from high and low latitudes, 

 and even from high and low altitudes. This very interest- 

 ing subject of inquiry he further elucidates in this way : 

 he found among the extensive material in the Smithsonian 

 Institution that specimens of birds from further north than 

 40 Latitude (taken as an average line of demarcation) were 

 larger than those born in the more southern localities ; and 

 this even was the case with indigenous species ; moreover, 

 there appeared in the birds of Florida a strange tendency 

 to " absolute increase of the size of the bill, even with di- 

 minution in general bulk." This would almost indicate a 

 deterioration of race, such as I have remarked in certain wild 

 animals occupying the torrid plains of India and the same 

 species on the Himalayas. Again, whilst "the Florida birds 

 were found to have larger bills than their more northern 

 brethren, several of the birds of the middle and western 

 regions of North America show an increase in the length 

 of the tail, as compared with the same or allied species 

 in the east." With such illustrations, on the authority of 

 one so competent to observe as the learned Secretary of 

 the Smithsonian Institution, we cannot fail to regard these 

 and like variations in size and proportion of the organ- 

 isms referred to, as valuable exponents in determining 

 species. Such important inquiries, extending over entire 

 continents, and pursued in a true philosophical spirit, will 



