the stronger ones would continue to live under stress- 

 ing conditions which would undoubtedly result in 

 changes in them after a sufficient number of genera- 

 tions. Naturally all of these modifications bring about 

 a change in the distribution of animal forms. The 

 movement of soil from the north, by the glacier, made 

 conditions favorable for the southward migration of 

 northerly plants and animals, and we find many things, 

 notably plants and insects in northwestern Pennsyl- 

 vania, that are not to be found elsewhere in the state. 



Other factors also affect the distribution of life. 

 Nature has established barriers to migration in the 

 forms of mountains, lakes, oceans, etc., over which some 

 forms can not pass. The fauna west of the Rocky Moun- 

 tains, for instance, differs from that of the eastern slope, 

 because many creatures can not negotiate the great 

 heights. Even the birds, which are better equipped 

 for migration than most other forms, have found the 

 mountains to be an impassable barrier and only a few 

 of them are able to fly across. On the other hand, wind, 

 rivers and water gaps are natural highways over which 

 the distribution of life is facilitated. 



Distribution within a limited area, such as in the 

 state of Pennsylvania under modern conditions, is a 

 rather artificial thing. The result of dissemination, 

 through common carriers and through the activities of 

 humans themselves, is rapidly being felt, particularly 

 in respect to destructive organisms. Deforestation, 

 industrialization and the tendency toward the popula- 

 tion of all available land areas, has had, and will con- 

 tinue to have, a pronounced effect upon the distribu- 

 tion of wild life. All of these factors tend to upset 

 the balance of Nature and to reduce the numbers of 

 wild creatures. 



