THE ALLIES OF THE PIGEON 



401 



Geographical Distribution of Birds. Because of their strongly 

 developed powers of flight, birds have fewer barriers to their 

 distribution than exist for most other animals. As a result 

 the same species frequently occur over large areas. In the 

 United States, the Rocky Mountains are a distinct bar- 

 rier to many species of birds. Except where barriers exist, 

 regions offering similar conditions for life may have the 

 same species of birds even though they be widely separated 

 geographically. Thus the en- 

 tire area east of the Rocky 

 Mountains has practically the 

 same species of bird inhabit- 

 ants, while west of the Rockies 

 there are many species differ- 

 ent from those of the Eastern 

 United States. 



When a given species of bird 

 occurs over a wide area, there 

 are often minor differences such 

 as of color or of size which dis- 

 tinguish the individuals of one 

 region from those of another. 

 These differences mark off the 



varieties within a species, as mentioned on page 108. Thus, 

 though song sparrows occur from one coast to the other, 

 they represent a considerable number of varieties. 



Birds of Past Ages. The earliest remains of birds of which 

 we have any knowledge come from the Age of Reptiles. 

 The oldest of these remains is the famous fossil known as 

 Archseop'teryx (Fig. 211), two specimens of which have been 

 found in Bavaria. The ancestry of all known birds is there- 

 fore to be traced back, at least so far as our knowledge goes, 

 to these two specimens. Archseopteryx was a land bird 

 about the size of a crow, probably arboreal in its habits, 



Fig. 211. Reconstruction draw- 

 ing of the primitive fossil bird, 

 Archseopteryx 



After Pycraft 



