ch.vi.] THE EVOLUTION OF THE EARTH. 399 



tion, entailing increased rotatory velocity, must end in the 

 segregation of the equatorial zone from the rest of the 

 system. This principle is exemplified, on a diminutive scale, 

 in the artificial evolution of a system of oil-globules, whereby 

 M. Plateau has imitated the evolution of the planets. To the 

 resulting equilibration between gravity and the centrifugal 

 tendency at the place where the detachment occurred, is due 

 the permanence and definiteness of the structural different- 

 iation. Owing to these conditions, and to its enormous size, 

 implying great power of condensation along with the very 

 slow dissipation of the heat generated by the condensation, 

 the integration of our genetic nebula has been compatible 

 with the retention of much relative motion of parts. And 

 here accordingly, as in all cases where there is a considerable 

 retention of internal motion, the secondary rearrangements 

 characteristic of Evolution have been conspicuously mani- 

 fested. 



In the evolution of our earth, regarded by itself, we have also 

 to notice a very decided progress in determinate multiformity, 

 even without taking into the account that specialized group of 

 terrestrial phenomena which we distinguish as organic. Here 

 there have been two conditions favourable to the retention of 

 enough motion to allow considerable secondary rearrangement 

 of parts. In the first place, the great size of the earth has 

 prevented it from parting too rapidly with the heat generated 

 during its condensation ; and since the early formation of a 

 solid, poorly-conducting crust, the loss from radiation would 

 seem to have been very gradual. The importance of this 

 circumstance may best be appreciated by remembering the 

 very different career of the moon, as indicated in the foregoing 

 chapter. The disappearance of igneous and aqueous agencies 

 on the moon implies the cessation of structural rearrangement 

 there at this early date; 1 and when we sought for an explana- 



1 This statement must be taken, however, with some qualification, Sea 

 »bove, p. 380. 



