THE OCEAN AND THE LAND 1 99 



Three possible ways of obtaining food instead of one means 

 a corresponding diversity in the fundamental structure of the 

 animals involved; but the relative uniformity of the physical 

 and chemical conditions in the sea permits the existence of 

 these major t^-pes with relatively slight subdivision. 



Increase in the height above sea level of the land means not 

 only a gradual decrease in the temperature, but also changes 

 in humidity, in radiation, in air pressure, and in other factors 

 bearing directly on the animal life. The higher the land the 

 more difficult are the conditions which animals must meet, and 

 furthermore the higher the land the greater is the distance to 

 any other similar region and hence the greater the difficulties 

 in the way of the dissemination of the various high land and 

 mountain t}^es. 



In the sea the temperatures in any given region and at any 

 given depth are only slightly variable, if at all. With increasing 

 depth the temperatures become gradually lower and more uni- 

 form, there is an increase in the gas content of the water, and 

 the distances between similar regions become less and less. 



On land the optimum conditions for both plants and animals 

 are in the moister regions of the tropics where the temperatures 

 are high, but not too high, and constant. The tropic lowlands 

 are the paradise of the naturalist, and from this region the 

 variety and bulk of living things decrease north and south, 

 and with increase in elevation. The frost hne is an important 

 factor on the land in hmiting the ranges both of plants and an- 

 imals, for by freezing, water is converted to a form in which it 

 cannot be used by plants or animals without a considerable ex- 

 penditure of energy. 



In the sea the colder the water the greater the amount of 

 gases dissolved in it, and consequently the greater the bulk of 

 living things it can support. The optimum conditions for both 

 plants and animals in the sea are found in the Arctic and 

 Antarctic regions in the summer when the sun is at its highest 

 and in the cold currents flowing outward from the polar seas. 

 As the water warms toward the equator life becomes less 



