320 THE LAW OF EVOLUTION. 



must have existed as vapour. This enormous volume of 

 disintegrated liquid became integrated as fast as the dissi- 

 jDation of the Earth's contained motion allowed; leaving, at 

 length, a comparatively small portion unintegrated, which 

 would be far smaller but for the unceasing absorption of 

 molecular motion from the Sun. In the formation 



of the Earth's crust we have a similar change similarly 

 caused. The passage from a thin solid film, everywhere 

 fissured and moveable on the subjacent molten matter, to a 

 crust so thick and strong as to be but now and then very 

 slightly dislocated by disturbing forces, illustrates the pro- 

 cess. And while, in this superficial solidification, we see 

 under one form how concentration accompanies loss of con- 

 tained motion, we see it under another form in that diminu- 

 tion of the Earth's bulk implied by superficial corrugation. 



Local or secondary integrations have advanced along 

 with this general integration. A molten spheroid merely 

 skinned over with solid matter, could have presented noth- 

 ing beyond small patches of land and water. Differences of 

 elevation great enough to form islands of considerable size, 

 imply a crust of some rigidity; and only as the crust grew 

 thick could the land be united into continents divided by 

 oceans. So, too, with the more striking elevations. The 

 collapse of a thin crust round its cooling and contracting 

 contents, would throw it into low ridges: it must have 

 acquired a relatively great depth and strength before ex- 

 tensive mountain systems of vast elevation became pos- 

 sible. In sedimentary changes, also, a like pro- 

 gress is inferable. Denudation acting on the small surfaces 

 exposed during early stages, would produce but small local 

 deposits. The collection of detritus into strata of great 

 extent, and the union of such strata into extensive " sys- 

 tems," imply wide surfaces of land and water, as well 

 as subsidences great, in both area and depth; whence it 

 follows that integrations of this order must have grown 

 more pronounced as the Earth's crust thickened. 



