§ 49. CLIMATE OF THE PALAEOZOIC AGES. 743 



utterly at variance with that which has prevailed during 

 the human epoch — the mind naturally endeavours to pene- 

 trate the mystery by a reference to physical causes extra- 

 neous to our planet. But as yet. astronomy has afforded 

 no satisfactory elucidation of the subject. 



A variation in the eccentricity of the earth's orbit, and 

 a change in the position of the tropical zone, from the pre- 

 cession of the equinox — both changes, which though ex- 

 tremely slow, are appreciable — have been brought forward 

 to account for the phenomena under review. 



From the diminution of the eccentricity of the earth's orbit round 

 the sun. by which the ellipse is in a state of approach to a circle, the 

 annual average of solar radiation is on the decrease ; and therefore, as 

 a general cause, and one affecting the mean temperature of the whole 

 globe, and the effect of which is both inevitable and susceptible of 

 exact estimation, it is deserving consideration. Sir John Herschel 

 states, u that an amount of variation in the eccentricity of the orbit, 

 which we need not hesitate to admit, at least provisionally, as a pos- 

 sible one, may be productive of considerable diversity of climate, and 

 may operate during great periods of time, either to mitigate or to 

 exaggerate the difference of winter and summer temperatures, so as 

 to produce alternately in the same latitude of either hemisphere a 

 perpetual spring, or the extreme vicissitudes of a burning summer 

 and a rigorous winter. "* 



In assuming a temperature in northern regions sufficient to support a 

 tropical vegetation, it must, too, be borne in mind, that light is as in- 

 dispensable as heat for the luxuriant growth of arborescent ferns, palms, 

 cycadeae, kc: and, by analogy, for the gigantic club-mosses and ferns 

 of the carboniferous period. The absence of light for weeks, or months, 

 would probably be fatal to most tropical forms of vegetation. It is 

 therefore as necessary to account for the presence of light as for a high 

 temperature in the northern regions, where the fossil plants indicate 

 the former genial influence of a hot climate, and sunny skies, during 

 the carboniferous epoch. 



To account for the existence of regions capable of supporting such 

 a flora as that of the coal-measure, in northern latitudes, a theory, 



* On the Astronomical Causes which may influence Geological 

 Phenomena. Geol. Trans, vol. iii. p. 293. 



