GENERAL LAWS OF ORIGIN AND MIGRATION. 249 



4. It has been supposed that the earth may have at 

 different times traversed more or less heated zones of 

 space, giving alternations of warm and cold temperature. 

 No such differences in space are, however, known, nor 

 does there seem any good ground for imagining their ex- 

 istence. 



5. The heat of the sun is known to be variable, and , 

 the eleven years' period of sun-spots has recently attracted 

 much attention as producing appreciable effects on the 

 seasons. There may possibly be longer cycles of solar 

 energy, or the sun may be liable, like some variable stars, 

 to paroxysms of increased energy. Such changes are 

 possible, and may fairly be taken into the account, pro- 

 vided that we fail to find known causes sufficient to ac- 

 count for the phenomena. 



Of well-known causes there seem to be but three. 

 These are : First, that urged by Lyell — viz., the varying 

 distribution of land and water along with that of marine 

 currents ; secondly, the varying eccentricity of the earth's 

 orbit, along with the precession of the equinoxes, and the 

 effects of this on oceanic circulation, as illustrated by 

 Croll ; thirdly, the different conditions of the earth's 

 atmosphere with reference to radiation, as argued by Tyn- 

 dall and Hunt. As these causes are all founded on known 

 facts, and not exclusive of each other, we may consider 

 them together. I shall take the Lyellian theory first, re- 

 garding it as the most important, and the best supported 

 by geological facts. 



We know that the present distribution of land and 

 water greatly influences climate, more especially by af- 

 fecting that of the ocean currents and of the winds, and 

 by the different action of land as compared with water in 

 the reception and radiation of heat. The present distri- 

 bution of land gives a large predominance to the arctic 

 and sub-arctic regions, as compared with the equatorial 

 and with the antarctic ; and we might readily imagine 

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