No. 624] MIGRATION A FACTOR IN EVOLUTION 



tively small area, isolation, even though they may over- 

 flow into streams. Inland waters are in general rela- 

 tively ephemeral in character compared with marine 

 waters, because with progressive erosion of the land they 

 tend to become extinct through deposition and ultimate 

 drainage. 



(c) Running Waters,— The transporting power of 

 running water is easily evident. The constant direction 

 of flow, its duration (as some drainage lines are of ex- 

 treme antiquity), and repeated transportation, have sub- 

 jected animals again and again to new conditions, and 

 carried them to new localities. Streams transport both 

 land and aquatic animals and by their persistence, activ- 

 ity, and the thoroughgoing fashion in which they work over 

 the land surface, are one of the most powerful agencies 

 of transportation. Streams undergo changes depending 

 on the dynamic status of the stream. The greater stress 

 to which the stream is subjected by uplift, the greater its 

 v^elocity and its relative transporting power, and the 

 nearer it erodes to base-level, the less current and relative 

 transporting power it possesses. Most animals counter- 

 act the transporting power of the stream by definite re- 

 sponses to the current, and thus maintain their position 

 and are not carried away. 



4. Litho spheric Agencies in Transportation 

 The lithosphere includes the solid earth, which to the 

 ordinary mind is the ideal of stability. The transport- 

 ing power of the solid is, however, usually at a very slow 

 rate, but this is not always the case, because of the sud- 

 denness of fracture. The solid ice of the glacier moves 

 slowly and yet travels long distances, but usually does 

 not transport an abundance of animals. Avalanches 

 move with greater speed, but they operate in rather 

 limited areas. Landslides transport, slowly or rapidly, 

 large masses of land containing animals. All of these 

 processes are dominated by gravity, and tend to trans- 

 port animals from a higher to a lower altitude. Perhaps 



