THE NEW EVOLUTION 



est variety of life is found where conditions vary more 

 or less extensively, but not too widely, from the 

 optimum. 



On land the greatest variety in the plants and ani- 

 mals is to be found in the less humid and cooler 

 regions of the tropics where seasonal change is slight 

 or absent, but the temperature and humidity vary 

 considerably between the night and day, and in those 

 regions in the tropics with a more or less marked, 

 though not severe, dry season. 



Among the animals of the sea the greatest variety 

 occurs on tropical and subtropical coasts at moderate 

 depths where the temperature of the water is just 

 about half way between the temperature of the 

 abysses and the temperature at the shore line. This 

 region is a sort of meeting ground wherein are found 

 many representatives of abyssal types which rise no 

 higher, and also representatives of many shore line 

 forms which descend no lower, while at the same time 

 very many different creatures occur in this band alone. 

 Much the same conditions are to be found on land in 

 mountainous regions in the tropics. 



There is still to be considered the region producing 

 the greatest amount of life, that is, the maximum 

 amount of living substance. One might suppose that 

 this would be the region where the greatest amount 

 of disintegration of the basic rocks is constantly 

 taking place. And so it is. On land the maximum 

 amount of living substance is found in the northern 

 hemisphere between about the Tropic of Cancer and 

 about 60° north latitude where there is the greatest 



[56] 



