12 LIFE ON THE EARTH. 



annual mean temperature, the extremes of yearly and 

 daily heat and cold, and the humidity of the atmo- 

 sphere, in a good degree dependent on temperature, 

 are conditions to which life in general, and special 

 forms of life in particular, are adjusted. 



For example, proceeding from the equator to 

 the north, along the land in the new or the old 

 world, we find the number of the forms of life con- 

 tinually grow less and less. According to an esti- 

 mate of some date, if we count in the warm zones 

 of Asia the plants which range northwards from the 

 equator with a temperature of 80, so as to reach the 

 latitude where the annual mean temperature of 64 

 prevails, we shall find 4500 species ; between 64 and 

 48 the number is reduced to 1500, while between 

 32 and the total sinks to 500, till in Walden 

 Island, lat. 80J north, only ten species occur ; and 

 finally, the whole series becomes extinct. 



So if we estimate the land mammalia of the Tropical 

 zone at 800 species, the proportionate number for 

 the Temperate zone is 200, and for the Polar zone 

 twenty. But the reverse holds in regard to the 

 Cetacea, which increase towards the Polar oceans. 



It may be further observed that the earth seems 

 to have the two extremes of life, the hot extreme 

 in the deserts of Africa, the cold extreme toward 

 either pole, and toward the summits of high moun- 



