CLASSIFICATION AND DISTRIBUTION 381 



for considerable distances; but for most land animals the ocean 

 is an effectual barrier. 



The land. For marine animals, the land proves to be an 

 effective barrier. Although the conditions are essentially the 

 same on both sides of the isthmus of Panama, the animals on 

 the two sides of the isthmus are different, the narrow land 

 barrier being sufficient to prevent animals from crossing from 

 sea to sea. Land is also a fairly effectual barrier in preventing 

 the water animals of one river system from passing to another. 

 The inhabitants of the river may distribute themselves over 

 a wide territory, but they are usually unable to pass from one 

 watershed to another, except as they may be carried by inci- 

 dental means. 



Mountains. The high mountain ranges are perhaps the 

 most effectual barriers of all. Practically no animal or plant 

 is able to cross over the higher mountain ranges. Hence it 

 sometimes happens that the animals and plants upon the two 

 slopes of high mountains may be quite different, even though 

 the climatic conditions on the two sides are essentially the same. 



Climate. Each animal and plant is able to live only in cer- 

 tain conditions of climate. Hence the climate of a territory is 

 a determining factor in regulating its inhabitants. In their 

 distribution, animals and plants are frequently completely 

 checked when they reach territories in which the climate is 

 unadapted to them. This may be the result of several different 

 factors. 



1. Water. The absence of water is a most effectual barrier 

 to the distribution of either animals or plants. Deserts are 

 uninhabited by any form of life, since no protoplasm can exist 

 without water. Although most forms of life need a moist 

 climate, some prefer one that is moderately dry and cannot 

 live in moist territories. Deserts and semi-deserts will, there- 

 fore, be barriers for the greater number of animals and plants, 

 while moist climates will be effectual barriers for the type of 

 organism which prefers a semi-dry climate. 



