THE LAW OF EVOLUTION. 319 



each partially-independent part, has been thus integrating, 

 there has been the further integration implied by increas- 

 ing combination among the parts. The satellites of each 

 planet are linked with their primary into a balanced cluster ; 

 while the planets and their satellites form with the Sun, a 

 compound group of which the members are more strongly 

 bound up with one another than were the far-spread por- 

 tions of the nebulous medium out of which they arose. 



Even apart from the nebular hypothesis, the Solar Sys- 

 tem furnishes evidence having a like general meaning. Not 

 to make much of the meteoric matter perpetually being 

 added to the mass of the Earth, and probably to the masses of 

 other planets, as well as, in larger quantities, to the mass of 

 the Sun, it will suffice to name two generally-admitted 

 instances. The one is the appreciable retardation of comets 

 by the ethereal medium, and the inferred retardation of 

 planets a process which, in time, must bring comets, and 

 eventually planets, into the Sun. The other is the Sun's 

 still-continued loss of motion in the shape of radiated heat; 

 accompanying the still-continued integration of his mass. 



109. To geologic evolution we pass without break 

 from the evolution which, for convenience, we separate as 

 astronomic. The history of the Earth, as traced out from 

 the structure of its crust, carries us back to that molten state 

 which the nebular hypothesis implies ; and, as before pointed 

 out ( 69), the changes classed as igneous are the accom- 

 paniments of the progressing consolidation of the Earth's 

 substance and accompanying loss of its contained motion. 

 Both the general and the local effects may be briefly exem- 

 plified. 



Leaving behind the period when the more volatile ele- 

 ments now existing as solids were kept by the high tem- 

 perature in a gaseous form, we may begin with the fact 

 that until the Earth's surface had cooled down below 212, 

 the vast mass of water at present covering three-fifths of it, 



