352 THE EVOLUTION OF LIFE. 



to this brief period, as tlie total amount of structural change 

 seen in the evolution of a complex organism out of a simple 

 genu, bears to that vast period during which living forms 

 have existed on the Earth. 



We have, indeed, much the same kind and quantity of 

 direct evidence that all organic beings have gradually arisen 

 through the actions of natural causes, which we have that all 

 the structural complexities of the Earth's crust have arisen 

 through the actions of natural causes. It may, I think, be 

 fairly said, that between the known modifications undergone by 

 organisms, and the totality of modifications displayed in their 

 structures, there is no greater disproportion than between the 

 geological changes which have been witnessed, and the to- 

 tality of geological changes supposed to be similarly caused. 

 Here and there are pointed out sedimentary deposits now 

 slowly taking place. At this place, it is proved that a shore 

 has been encroached on by the sea to a considerable extent 

 within recorded times ; and at another place, an estuary is 

 known to have become shallower within the space of some 

 generations. In one region a general upheaval is going on 

 at the rate of a few feet in a century; while in another 

 region occasional earthquakes are shown to cause slight 

 variations of level. Appreciable amounts of denudation by 

 water are visible in some localities ; and in other localities 

 glaciers are detected in the act of grinding down the rocky sur- 

 faces over which they glide. But the changes thus instanced, 

 are infinitesimal compared with the aggregate of changes to 

 which the Earth's crust testifies, even in its still extant sys- 

 tems of strata. If, then, from the small changes now being 

 wrought on the Earth's crust by natural agencies, we may 

 legitimately conclude that by such natural agencies acting 

 through vast epochs, all the structural complexities of the 

 Earth's crust have been produced; may we not from the 

 small known modifications produced in races of organisms 

 by natural agencies, similarly infer that from natural agen- 



