MODIFICATION OF THEORY EXAMINED. 103 



mine the character of their operations. For example, 

 the position of the winter solstice in relation to the 

 aphelion or to the perihelion, during a high state of 

 eccentricity, determines whether the physical agencies 

 will produce on a given hemisphere a glacial or a 

 warm condition of climate ; while precession deter- 

 mines which of the two hemispheres shall be the 

 glaciated and which the warm. In one respect we 

 may say that the astronomical causes produce glacia- 

 tion by means of the physical agencies. 



The geographical conditions, however, cannot pro- 

 perly be considered to be causes in the sense in which 

 the astronomical and physical are. They are more 

 properly conditions to the production of a glacial 

 epoch than causes. They cannot be said to act in the 

 production of glaciation. They are rather permanent 

 and passive conditions enabling the active causes to 

 produce their required effects. Had the glacial epoch 

 resulted from the elevation of the land, as some 

 geologists suppose, then this elevation might properly 

 be said to have been the cause of the glacial epoch ; 

 but the glacial epoch was produced by no such means, 

 nor by any change in the physical geography of the 

 globe. A certain geographical condition of things 

 was, of course, requisite in order to the effective 

 operation of the astronomical and physical causes. 

 This condition existed at the time of the glacial epoch ; 

 and it is only in this sense that that epoch can be 

 referred to anything geographical. 



It is true that a cause, as Sir William Hamilton 

 states, may be defined as " all that without which the 

 effect would not happen ;" but this is far too general 

 an expression of cause for practical purposes. We 

 therefore fix on the particular antecedent or antece- 

 dents, through the activity of which the event is 



