THE EVOLUTION OF THE EARTH 



is very marked, though by no means so pro- 

 minent as in the case of organic evolution. This 

 increase in determinate multiformity, such as it 

 is, is due to the special mechanical principle 

 that in any rotating system of particles, regarded 

 as practically isolated, a steady concentration, 

 entailing increased rotatory velocity, must end 

 in the segregation of the equatorial zone from 

 the rest of the system. This principle is ex- 

 emplified, 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 

 differentiation. Owing to these conditions, and 

 to its enormous size, implying great power of 

 condensation along with the very slow dissipa- 

 tion 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 rearrange- 

 ments characteristic of Evolution have been con- 

 spicuously manifested. 



In the evolution of our earth, regarded by 

 itself, we have also to notice a very decided 

 progress in determinate multiformity, even with- 

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