160 EXPLANATIONS. 



space of this higher order in point of magnitude, ive 

 may, probably, compare such an interval of time as 

 that which divides the human epochfrom the origin of 

 the coralline limestone, over which the Niagara is pre- 

 cipitated at the Falls. Many have been the suc- 

 cessive revolutions in organic life, and many the 

 vicissitudes in the physical geography of the globe, 

 and often has sea been converted into land, since 

 that rock was formed. The Alps, the Pyrenees, 

 the Himalaya, have not only begun to exist as 

 lofty mountain chains, but the solid materials of 

 which they are composed have been slowly ela- 

 borated beneath the sea, within the stupendous 

 interval of ages here alluded to."* 



If time, to anything like the amount here in- 

 sisted on, have really elapsed between the com- 

 mencement of life and its attaining its highest 

 forms, we must see that the space comprised by 

 the life of an individual, or even that longer portion 

 during which mankind have been watching the 

 wonders of nature, is not sufficient to allow more 

 than a chance of any transition of species being 

 or having been observed, except perhaps in the 

 humble fields where, as was formerly remarked, 

 reproduction is most active and types least defined. 

 * Travels in North America, i. 52. 



