CAUSES OF CLIMATAL CHANGE 385 



(4) The effects of change of eccentricity and precession have 

 been so ably urged by Croll, and recently by Ball, and have so 

 strongly influenced the minds of those who are not working 

 geologists, that they deserve a more detailed notice. 



(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 in- 

 creased energy. Such changes are possible, but we have no 

 evidence of their occurrence, and they could not account for 

 periods of refrigeration of limited duration like the Glacial 

 age. 



(6) It has been supposed that the earth may have at dif- 

 ferent 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 existence. 



(7) The differences in the form and elevation of our conti- 

 nents, and in the consequent distribution of surfaces of different 

 absorbent and radiating power, and of the oceanic currents, are 

 known causes of climatal change, and have been referred to in 

 these papers as competent to account for many, at least, of the 

 phenomena. 



(8) Reference has already been made, in connection with the 

 distribution of plants, to the possibility that the primeval 

 atmosphere was richer in carbon than that of more modern 

 times, and that this might operate to produce diminution of 

 radiation, and consequent uniformity of temperature ; but this 

 cause could not have been efficient in the later geological 

 periods. 



There may thus be said to remain two theories of those 

 enumerated by Wood, to which more detailed consideration may 

 be given, namely, numbers four and seven, which may be named 



