Geographical Distribution 



177 



species have migrated into the United States. There are 

 several birds (jays and titmice) which are characteristic of 

 the pinon belt on San Francisco Mountain and a few mammals 

 (mice and ground squirrels), while several lizards come in 

 from the " Painted Desert" to the east. Two species of liz- 

 ards however appear to be characteristic of this zone, and 

 one (a horned toad) wanders up into the transition zone 

 above. 



CYPRESS SWAMP, ARKANSAS 

 Courtesy of the U. S. Bureau of Biological Survey. 



But temperature is not the sole factor regulating the dis- 

 tribution of animals and plants. Moisture, light, the char- 

 acter of the soil and the surface of the country these and 

 other factors, all work together and influence one another to 

 determine this distribution. Perhaps the most important of 

 these is moisture. The traveller passing across the United 

 States from east to west, finds himself at the outset of his 

 journey on the moist plain of the Atlantic Coast, a region 

 which has but recently (in geologic time) been raised above 

 the level of the sea. Along the New England Coast, the cold 

 Labrador current in its southward sweep, produces the fogs 

 which so often shroud these shores, while the Florida penin- 

 sula receives the moisture laden winds from both the Atlantic 



