DISTRIBUTION OF ANIMALS. 375 



369. Fundamental Faunas. There are certain funda- 

 mental conditions which determine life, irrespective of 

 the place on the surface of the earth. These conditions 

 are more important than temperature, or latitude, or 

 altitude, or the differences between forest and prairie. 

 The faunas dependent upon these conditions are the 

 marine or salt-water faunas, the fresh-water faunas, the 

 land faunas, and the aerial faunas. The pond life of 

 central Africa is more like the pond life of America than 

 the life in the pond is to that on the land three feet away. 

 The greatest of all differences are between the life in water 

 and the life on land. These differences depend primarily 

 upon the method of breathing. 



On top of these great features, which determine the 

 life, are others of less importance. For example, the 

 animals that live at the margins of the seas differ con- 

 siderably from those out on the high seas; and both of 

 these are very different from those at the bottoms of the 

 deep seas. Similarly we have seen that the varying 

 conditions on land produce their effect on the nature of 

 the life. 



370. The Course of Development of Faunas. If the 



evolutionary view is correct, animals have not always 

 been distributed as they are to-day. This condition is 

 itself an evolution through the ages. It is the belief that 

 the first life was aquatic, and marine rather than fresh 

 water. Whether it was out in the high seas or near to 

 shore is a matter of speculation. From this original 

 position organisms spread and became adjusted to the 

 other marine conditions. Gradually some became ad- 

 justed to brackish water at the mouths of rivers and 

 finally to the rivers themselves. From the sea or from 



