254 GENERALIZATIONS. 



of 1956 metres (or 2139 yards) in the Speer, and of 1800 metres 

 (or 1968*5 yards) in the Eigi. And yet it is extremely probable 

 that the time between the formation of the first or Tongrian 

 stage of the Miocene and the fifth or CEningian stage of the 

 Miocene was much longer than the Pliocene period. The stra- 

 tification of the Swiss Alps shows that, in the history of their 

 development, periods of comparative repose have alternated with 

 times of deep-seated transformations. 



It may now be asked, In what relation do geological periods 

 stand to the eras of human history ? and can the duration of the 

 various geological periods and the intervals up to the present 

 time be expressed in definite numbers ? Before answering these 

 questions let us try to ascertain what measure can be applied to 

 the examination of these relations. Man employs, as a measure 

 of time, the length of his own life between his birth and his 

 death ; and the influence that this exerts upon all his ideas of 

 nature has been shown by the academician Charles de Baer, in 

 the following ingenious manner, easy to be understood. De 

 Baer takes for his basis that the duration of man's life may be 

 reckoned at eighty years (or about 29,000 days) : he supposes 

 that this number is reduced to the thousandth part, or 29 days, 

 and that simultaneously the pulse becomes proportionably more 

 frequent, and the conception of external impressions * more 



* The conception of an external impression is determined by the time that 

 elapses between the sensation of the impression and its reception by the mind. 

 Thus, as has been ascertained by direct experiment, the impression produced 

 upon the retina of the eye in Man requires iV" e~ f a second to nmke its way 

 to the brain, and to be there transmitted to the mind. The duration of this 

 movement must exert much influence upon the general conception of the ex- 

 ternal world j and there would be a different conception if these external im- 

 pressions were only transmitted to the mind in a minute, or in ten minutes, 

 &c. and especially as there would also be a change with respect to the trans- 

 mission of impressions due to the constant phenomena of the undulations of 

 sound and of light. 



[Some consideration may also be given to the power of the human mind 

 over space and time. Cowper beautifully alludes to the rapidity of man's 

 thought in his verses, supposed to have been written by Alexander Selkirk 

 during his solitary abode in the island of Juan Fernandez : 



