184 LyelVs Principles of Geology. 



derable extent ; and as for principal agents, such as humi- 

 dity, it is easily supplied when deficient. No arguments 

 can be fairly drawn from cultivated plants, because of the 

 influence of acclimation (though this is perhaps denied by 

 the Zoological Kingdom), and of the artificial modes resorted 

 to imitate the contingencies to which the plant is in a state 

 of nature exposed. 



The diffusion of light is unequal in actual amount, its 

 rapidity is erroneous. Query, how does Humboldt prove that 

 it is less owing to the absence of heat, than to the want of 

 sufficient solar light that the vine does not ripen its fruit 

 beneath the foggy skies of Normandy ? Because the existence 

 of a fog diminishes heat as much as it does light, or per- 

 haps he means abstractly solar light. 



As both heat and humidity decrease as we ascend either 

 in elevation or latitude, so that at certain points throughout 

 the world an arctic flora exists, we are led to a comparison 

 of the mutual influence of these agents, i. e. between the 

 arctic regions of mountains, and those towards the poles. The 

 recurrence of an arctic flora at all sufficiently elevated points 

 is a strong argument against the supposed great action of 

 mere density of the air, for we have similar plants appearing 

 on spots where the density is diminished four-fifths ! and 

 where it exists at its mean. But there may be a striking 

 similarity between amount of electricity, and other little 

 known agents. The occurrence of arctic points is a striking 

 proof of the value of two causes, viz. elevation or high lati- 

 tude, and humidity. 



What correspondence is there in amount of solar light? 

 Compare the periods during which light exists at the poles 

 and in the tropics ? Does the length of the polar summer at 

 all compensate ? 



Notice the fact that within the tropics of Asia, Africa, 

 and America, the genera are similar, but the species rarely 

 the same, 



