PLANETARY EVOLUTION 



tern, these thinkers proved that the mere con- 

 traction of such a mass must inevitably have 

 brought about just the state of things which we 

 now find. Let us observe some of the processes 

 which must have taken place in this nebulous 

 mass. 



Note first that we are obliged to accredit the 

 various parts of this genetic neimla with mo- 

 tions bearing some reference to a common cen- 

 tre of gravity ; for the rotation of the resulting 

 system must have had an equivalent amount of 

 motion for its antecedent, and it is a well-known 

 theorem of mechanics that no system of bodies 

 can acquire a primordial rotation merely from 

 the interaction of its own parts. In making this 

 assumption, however, we are simply carrying 

 out the principle of the continuity of motion. 

 It is not necessary to suppose, in addition, that 

 all these motions primordially constituted a ro- 

 tation of the whole mass in one direction. Such 

 a hypothesis seems to me not only gratuitous, 

 but highly improbable. It is more likely that 

 these primeval motions took the shape of cur- 

 rents, now aiding and now opposing one an- 

 other, and determined hither and thither accord- 

 ing to local circumstances. In any case, such 

 indefiniteness of movement must finally end in 

 a definite rotation in one direction. For unless 

 the currents tending eastward are exactly bal- 

 anced by the currents tending westward — a sup- 

 1SS 



