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with undeniable clearness to the fact of the existence of a 
climate in the northern hemisphere differing widely from 
the present, and more analogous to that now existing in 
the tropics. The gigantic calamites, lepidodendra, sigil- 
larise, and stigmarise, fed by the rich soils which no previous 
vegetation had impoverished, and fostered by the uniform 
warmth and humidity of the climate, formed at that time 
a mass of vegetable life of which even our tropical forests 
convey but a faint idea. In endeavouring to account for 
this fluctuation of climate, I have been led, after a careful 
study of the evidences hitherto adduced, to reject the theory 
of astronomical causes being a principal or even a minor 
agent. The mean amount of solar radiation being in the 
inverse ratio of the minor axis of the sun's orbit, would 
vary, of course, with the eccentricity of that orbit ; but the 
lapse of years required before such eccentricity would be 
sufficiently great perceptibly to influence the climate of our 
planet, renders it difficult to conceive that geological changes 
of such magnitude could be effected by it. 
It was formerly supposed that the plants of the carboni- 
ferous era were so closely allied to tropical genera, and 
so much larger in size than the corresponding tribes now 
inhabiting equatorial latitudes, as to imply an extremely 
hot as well as humid and equable climate. Recent investi- 
gations have, however, shown so wide a difference between 
past and existing species, that we must rather infer the 
existence of a climate more equable, but essentially different 
from, rather than analogous to, any existing climate. The 
evidence strongly points to the existence of a more uniform 
and temperate climate, rather characterised by the absence 
of any severe cold, than by the presence of extreme heat. 
I am not now going to discuss the probable cause of such 
a change of climate, but I would express a strong convic- 
tion that the elevation and depression of large tracts of 
