ZOOGENESIS 



living substance, will be most abundant for each unit 

 of area — for instance per acre — where there is a maxi- 

 mum of water permanently in the liquid state, a 

 maximum of air, and a maximum of food. 



Thus on land the optimum conditions for the 

 greatest development of plant and animal life, so far 

 as concerns mere bulk alone, are to be found in the 

 moister regions of the tropics where the rains are not 

 so heavy as to be destructive by the weight of water 

 falling, and the temperature is high and constant. 



But in the sea these factors find their most perfect 

 balance in a region wholly different. Here the chief 

 problem is securing a sufficiency of air. Now the 

 colder water gets the greater the amount of air it is 

 capable of holding in solution. Everyone has no- 

 ticed that in a glass of cold water standing in a room 

 bubbles of air appear on the sides and bottom as the 

 water warms. So in the sea the optimum conditions 

 for both plant and animal life are in the coldest oceans, 

 in the polar seas in the summer time when the sun is 

 at its highest, and in the cold currents flowing out 

 from these. 



In the hot wet portions of the tropics chemical dis- 

 integration of the rocks takes place with great rapid- 

 ity, continually replenishing the mineral constituents 

 in the food supply of the endemic plants. In the cold 

 polar seas the dead organic matter either in suspension 

 in the water or lying on the bottom is preserved, as 

 in an ice-box, for the longest time. 



So on land food supplies, in the form of new mate- 

 rials made available for use by plants, are especially 



[53] 



