414 INFLUENCE OF 



elevated 20 or 30, as he maintains ; it is still 

 evident, that the polar regions would be excluded 

 from the sun several months in the year, during 

 which time, all vegetation would be arrested ; 

 for it ceases whenever the mercury sinks below 

 52, according to Humboldt. It is therefore evi- 

 dent, that such a climatic condition of the earth 

 would be wholly incompatible with the existence 

 of a tropical vegetation, which is known to require 

 a mean temperature of about 80 throughout the 

 year. 



Among all the causes which modify the tem- 

 perature of our planet, the most influential by far 

 is the obliquity of the earth's axis. In reality, 

 this condition determines chiefly, the vast differ- 

 ence between the climate of the tropical, tem- 

 perate, and frigid zones, together with all the 

 varieties of season. The question therefore natu- 

 rally arises, whether all the great changes of 

 climate which the earth has undergone, may not 

 have been owing to modifications of the same 

 cause, such as variations in the inclination of 

 its axis, or even a gradual transposition of the 

 equator and poles ? In support of this natural 

 view of the subject, it may be observed, that the 

 successive generation and extinction of tropical 

 plants and animals in the higher latitudes, would 

 seem to require the repeated occurrence of such 

 astronomical changes, which in their turn, afford 

 a complete explanation of all the phenomena. 

 And that such changes are not inconsistent with 



