THE AGE OF THE EARTH AS AN ABODE FITTED FOR LIFE. 235 



edly opposed to the assumption of excessively high temperatures at 

 that stage. On the contrary, it has recently been urged from different 

 quarters that the early temperature of the sun's surface must have 

 been much lower than at present, and this is also implied in certain 

 statements of the address (p. 711, sec. 43). There are also grounds 

 for grave question as to the high temperature of the earth, as has 

 already been indicated. Under the revised forms of the nebular hypo- 

 thesis there seems no substantial reason for supposing that if the mat- 

 ter of the moon was once distributed in a ring about the earth, it could 

 maintain the gaseous condition throughout the stages of its condensa- 

 tion. The hypothesis therefore rests upon exceedingly doubtful prem- 

 ises, and upon exceedingly questionable deductions from these doubtful 

 premises. 



The fission hypothesis of George Darwin has recently replaced it in 

 favor, but the above quotation implies that even its founder does not 

 now rest much confidence in it. The objections to the theory are 

 several and grave. In the first place, the theory of the fission of a 

 celestial bod}' by high rotation, as worked out independently by Dar- 

 win and Poincare, requires that the separated bodies should not be 

 very greatly different in mass, i. e., the smaller body should not be 

 less than one-third the mass of the larger. But the mass of the moon 

 is but one-eightieth of that of the earth, and hence it lies far outside 

 the computed limits of applicability of the fission process. 



Another difficulty lies in the effect of tidal strain itself. George 

 DarAvin, in his recent work on The Tides (p. 259), assigns 11,000 miles 

 from the center of the earth as Roche's limit. This leaves a tract of 

 7,000 miles above the terrestrial surface within which the earth's tidal 

 force would be so great as to tear the moon to fragments, and, per- 

 haps, scatter these into the form of a ring. The rings of Saturn are 

 supposed to illustrate this form of intense tidal action. The escape 

 of the moon, even presuming it to have been separated from the earth, 

 would, therefore, have been jeopardized by its transformation into a 

 meteoroidal ring or swarm. If the fragments, after having been 

 torn apart, were still sufficiently affected b}' a minute tide to be car- 

 ried away from the earth in a slow spiral, the time occupied in passing 

 outward beyond Roche's limit must have been protracted; and, after 

 their escape from it into a zone where conditions not hostile to aggrega- 

 tion might, perhaps, have been afforded, there must probably have been 

 another protracted period before the aggregation of the moon would have 

 been sufficiently advanced to give it appreciable tidal effect upon the 

 earth. It remains, therefore, to be determined, if this hj^pothesis is 

 followed, at what stage in the evolution of the moon it was sufficiently 

 concentrated to assume effective tidal functions. This is a question 

 also applicable to the aggregation of the moon under the Laplacean 

 hypothesis, if it be modified so as to conform to the demands of mod- 



