264 



SCIENCE, 



[N. S. Vol. XXII. No. 557. 



in which the stability of the species of mo- 

 tion is tottering, so that the system pre- 

 sents the characteristic of a transitional 

 form, which we have seen to denote a 

 change of type or species iii a previous 

 case. 



In discussing the transformations of a 

 liquid planet we saw the tendency of the 

 single mass to divide into two portions, 

 and now we seem to reach a similar crisis 

 from the opposite end, when in retrospect 

 we trace back the system to two masses of 

 unequal sizes in close proximity with one 

 another. The argument almost carries 

 conviction with it, but I have necessarily 

 been compelled to pass over various doubt- 

 ful points. 



Time is wanting to consider other sub- 

 jects worthy of notice which arise out of 

 this problem, yet I wish to point out the 

 fact that tidal friction is competent to ex- 

 plain the eccentricity of an orbit, because 

 this conclusion has been applied in a man- 

 ner to which I shall have occasion to return 

 hereafter. 



If, as has been argued, tidal friction has 

 played so important a part in the history 

 of the earth and moon, it might be expected 

 that the like should be true of the other 

 planets and satellites, and of tlie planets 

 themselves in their relationship to the sun. 

 But numerical examination of the several 

 cases proves conclusively that this can not 

 have been the case. The relationship of 

 the moon to the earth is in fact quite ex- 

 ceptional in the solar system, and we have 

 still to rely on such theories as that of La- 

 place for the explanation of the main out- 

 lines of the solar system. 



I have not yet mentioned the time oc- 

 cupied by the sequence of events sketched 

 out in the various schemes of cosmogony, 

 and the question of cosmical time is a 

 thorny and controversial one. 



Our ideas are absolutely blank as to the 

 time requisite for the evolution either ac- 



cording to Laplace 's nebular hypothesis, or 

 the meteoritic theory. All we can assert is 

 that they demand enormous intervals of 

 time as estimated in years. 



The theory of tidal friction stands alone 

 amongst these evolutionary speculations in 

 that we can establish an exact but merely 

 relative time-scale for every stage of the 

 process. Although it is true that the value 

 in years of the unit of time remains un- 

 known, yet it is possible to determine a 

 period in years which must be shorter than 

 that in which the whole history is com- 

 prised. If at every moment since the birth 

 of the moon tidal friction had always been 

 at work in such a way as to produce the 

 greatest possible effect, then we should find 

 that sixty million years would be consumed 

 in this portion of evolutionary history. 

 The true period must be much greater, and 

 it does not seem unreasonable to suppose 

 that 500 to 1,000 million years may have 

 elapsed since the birth of the moon. Such 

 an estimate would not seem extravagant to 

 geologists who have, in various ways, made 

 exceedingly rough determinations of geo- 

 logical periods. 



As far as my knowledge goes I should 

 say that pure geology points to some period 

 intermediate between 50 and 1,000 mil- 

 lions of years, the upper limit being more 

 doubtful than the lower. Thus far we do 

 not find anything which renders the tidal 

 theory of evolution untenable. 



But the physicists have formed estimates 

 in other ways which, until recently, seemed 

 to demand in the most imperative manner 

 a far lower scale "of time. According to all 

 theories of cosmogony, the sun is a star 

 which became heated in the process of its 

 condensation from a condition of wide dis- 

 persion. When a meteoric stone falls into 

 the sun the arrest of its previous motion 

 gives rise to heat, just as the blow of a 

 horse 's shoe on a stone makes a spark. The 

 fall of countless meteoric stones, or the 



