52 
PHYSICAL GEOGRAPHY. 
due to solar light and heat, decreases, as a general 
rule, in a regular gradation from the tropics to the 
poles. It is owing to the greater or less exposure 
to the sun that we have such a variety of seasons 
and of climates in different latitudes, and no one can 
contemplate this wise arrangement without enlar- 
ging his conceptions in regard to the wisdom and 
benevolence of the Supreme Being. From numer- 
ous geological phenomena, it has been concluded 
that the temperature of the earth has undergone im- 
portant changes. This supposition rests chiefly on 
the discovery of vegetable and organic remains, im- 
bedded in situations where, from the want of a conge- 
nial temperature, such animals or vegetables would 
now be unable to exist. For example, tropical 
plants and fossil elephants are found in great quan- 
tities in high northern latitudes, in such a state of 
preservation that it seems next to impossible that 
they could have been drifted there from a distance. 
In accounting for this phenomenon, Mr. Herschel, the 
astronomer, considers that a diminution of the sur- 
face temperature might arise from a change in the 
elhpticity of the earth's orbit, which, though slowly, 
gradually becomes more circular. Mr. Lyell, again, 
supposes that this decrease of temperature might 
arise from such a variation in the relative position 
of land and w^ater, and in the elevation and form of 
land, as may cause the climate, in any given portion 
of the earth's surface, so to change, that a greater 
heat may precede a less heat, and the land be capa- 
ble of supporting the vegetables and animals of hot 
climates at one time, and incapable of doing so at 
another. It supposes a combination of external and 
internal causes ; the latter raising or depressing the 
land in the proper situations, the former supplying 
the necessary heat. This, though a highly ingenious 
theory, and not without some shght support from 
facts, is yot too improbable to meet with much fa- 
vour. It cannot be denied, however, that there is 
