200 THE INFLUENCE OF INANIMATE SURROUND INSS. 



CHAPTER VII. 



THE INFLUENCE OF WATER IN MOTION. 



In the previous chapter we saw that the distribution of animals 

 on our globe would be essentially modified by a change in the 

 proportions or in the chemical composition of air or of water. 

 If the air were deprived of the greater part of its oxygen, 

 only a very few species of animals could continue to live — only 

 those, that is to say, which could endure such a diminution of the 

 respirable oxygen contained in the air. If, on the other hand, 

 a larger quantity of oxygen could be added to the water than it 

 usually contains, it would appear probable that many animals 

 fitted for breathing in the air would be thus enabled to live 

 in the water if any other cause made such a change of habit 

 inevitable; consequently land animals would become aquatic. 

 It is not probable that such a complete change could now 

 ever actually take place ; but smaller changes in those con- 

 ditions of life might occur, in fact actually do occur. We 

 know, for instance, that, according to the direction of the wind, 

 the air at the surface of the earth is light or heavy, that it is of 

 difisrent density in low plains and on heights, and varies very 

 greatly in its composition ; it is difierent in the dwelling-places 

 of man and under the shady roof of forest trees, on the open sea 

 and in the Sahara or the boreal regions of the eastern and 

 western hemispheres ; and the percentage of moisture in the air 

 varies with the temperature and the prevailing winds. The 

 constituents of water are equally variable ; in lakes with marshy 

 shores they are not the same as in running brooks or rivers ; 

 they are different on a limestone soil and on sandstone ; the 

 amount of saline matter in solution (sensu strictiori) varies con- 



