602 General and Applied Biology 



differ in tundras, deserts, grasslands, and forests. Naturally, we would 

 expect to find different types of vegetation in each region. If the dis- 

 tribution of forests is studied in detail, it is apparent that different types 

 of trees are distributed in various parts of the continent, depending upon 

 the influential characteristics of the environment peculiar to each of 

 these forest areas. Could we expect the same type of forest in the far 

 north as we find in the tropics? Attempt to list the environmental char- 

 acteristics of each vegetation area and then use them to explain the 

 distribution of the types of vegetation peculiar to each of these regions. 

 In doing so, you will begin to appreciate how Nature functions in limit- 

 ing certain kinds of vegetation to specific areas. 



VI. GENERAL FACTORS INFLUENCING THE 

 DISTRIBUTION OF ORGANISMS 



Only a few of the more common and important factors can be con- 

 sidered in such a brief discussion as this. ( 1 ) The connection of regions 

 by the formation of bridges, such as the isthmus of Panama during the 

 later Miocene epoch (7,000,000 years ago) (Fig. 320) permitted migra- 

 tions of organisms in both directions. North and South America were 

 not connected in the early Miocene epoch of the Ccnozoic era as is 

 shown by fossil records of the prevailing faunas in these regions. (2) 

 Disconnection of regions by the formation of channels or straits, such 

 as Bering Strait between Asia and North America, prevented migrations 

 of certain animals. Fossil records in both regions show that migrations 

 occurred in both directions across the former land bridge before the 

 present strait was formed. (3) Glaciers may cause a lateral as well as 

 a vertical migration because of changes in temperatures, food supplies, 

 and places of protection. (4) The flora (plant population) of a region 

 affects either directly or indirectly the animal population of that region 

 as far as food, shelter, and protection are concerned. (5) The presence 

 of belligerent, antagonistic species may influence the distribution of cer- 

 tain species of organisms. The first appearance of certain species in 

 definite centers of dispersal may make it possible or impossible for later 

 migrations. In this case, priority rights of possession are determinative 

 factors. (6) When organisms are isolated from the main stock of species, 

 divergence is promoted in proportion to the degree of isolation and the 

 length of time isolated. This is in part accomplished by preventing new 

 types from being eliminated by interbreeding with the old. Isolation is 

 thus a factor in the process of descent with changes. According to Jor- 



