208 



ANIMALS OF LAND AND SEA 



In those long past ages which geologists call palaeozoic time 

 wave action on the sea shores was undoubtedly less violent 

 than now. The sea was much less salt, for less mineral matter 

 had been dissolved out of the soil and carried to it by the rivers. 

 Now the less salt there is in water the greater is the vapor 

 pressure and hence the more readily it evaporates, so that in 



ancient times when the seas were fresher 

 there must have been more water on the 

 surface of the land, in ponds and lakes 

 and in extensive marshes, as well as in 

 the air as clouds. 



Under these conditions, which obtained 

 when the insects were developing as 

 aquatic forms and amphibians were abun- 

 dant, it would have been less difficult for 

 them to enter the sea than it is at pres- 

 ent. 



Increasing saltness of the sea rendering 

 more and more difficult evaporation from 

 its surface would gradually increase the 

 amount of water held permanently in the 

 ocean basins, and decrease the amount 

 on the land and in the air. Shrinkage of 

 cloud areas on the land would mean in- 

 increase of sunlight and hence a marked 

 increase of temperature in the equatorial regions and a decrease 

 toward the poles. With this would come a rise in the strength 

 of winds and the formation of a breaker line along the shores, 

 gradually increasing in extent. 



The insects, frogs, toads and salamanders are animals ca- 

 pable of crawling, jumping, running or flying over land when 

 fully grown, but aquatic in the younger stages, truly amphib- 

 ious, though only found along the shores in very shallow water 

 and absent from the sea. 



A large and heterogeneous group of animals is very charac- 

 teristic of fresh water, though all its elements are represented 



Figs. 612-616. 



The shells of some bivalve 



molluscs. 



For explanations of the 

 figures see p. xxxi. 



