ch.v.] PLANETARY EVOLUTION* 357 



history of the solar system. Yet as Laplace has shown, it* 

 is practically impossible that such harmonious relations 

 should hold between the various members of the solar 

 system, unless those members have had a common origin. 



The clue to that common origin may be sought in facts 

 which are daily occurring before our very eyes. Every 

 member of our planetary system is constantly parting with 

 molecular motion in the shape of heat. Our earth is 

 incessantly pouring out heat into surrounding space ; and, 

 although the loss is temporarily made good by solar radia- 

 tion, it is not permanently made good, — as is proved by the 

 fact that during many millions of years the earth has been 

 slowly cooling. I do not refer to the often-cited fact that 

 the Arctic regions were once warm enough to maintain a 

 tropical vegetation ; for this high temperature may well have 

 been due to minor causes, such as the greater absorptive 

 power of the ancient atmosphere with its higher percentage 

 of carbonic acid and ozone. Nor need we insist upon the 

 alleged fact that extensive glaciation appears to have been 

 unknown until a comparatively late epoch ; although glacia- 

 tion, whether brought about by changes in the distribution 

 of land and sea or by a variation in the eccentricity of the 

 earth's orbit, certainly does seem to imply a progressive 

 dependence of the earth upon the supply of solar heat, due to 

 the lowering of its own proper temperature. Such facts, 

 however, are wholly inadequate to describe the primitive 

 heat of the earth. The flattening of the poles being con- 

 siderably greater than could have been produced by the 

 rotation of a globe originally solid on the surface, it follows 

 that the whole earth was formerly fluid. And this conclu- 

 sion, established by dynamical principles, is uniformly 

 corroborated by the observed facts of geology. Now the 

 fluidity of the entire earth, with its rocks and metals, 

 implies a heat sufficient to have kept the planet incandescent, 

 Bo that it must have shone with light of its own, like the 



