EECENT COSMOGONIES 213 



Uie Earth; and the name which he gives his theory is, 

 The Planetesimal Hypothesis. 



Now, there are two fundamental astronomical condi- 

 tions that especially concern geologists which must posi- 

 tively be supplied by any cosmological theory before it 

 can prove acceptable, and both of these concern the sun. 

 These conditions are ; first, that a source of heat be found 

 capable of maintaining him at practically an even aver- 

 age temperature over a period of the order of a thousand 

 million years, and, second, that the source so found shall 

 consistently explain the alternation of ice- and genial ages 

 which have occurred in the course of the earth's geologi- 

 cal history. 



On the first of these problems Doctor Chamberlin 

 does not, nor does he try to, throw any new light, but his 

 chief aim is avowedly to explain the alternation of the 

 cold and the warm periods. In the pursuit of this object, 

 he informs us, was he led "into the cosmogonic fens and 

 fogs," whence he emerged with this Planetesimal Hy- 

 pothesis as his quarry. 



Like Professor Bickerton, Doctor Chamberlin de- 

 rives the solar system from the accidental meeting of our 

 ancestral sun with another star ; but with this difference, 

 that whereas the former postulates actual collision, the 

 latter contemplates only "approach to within effective 

 tidal range. He pictures the sun as formerly a solitary 

 star, moving under the head of its so-called proper 

 motion, being fortuitously met or overtaken by just an- 

 other such star as himself. Beginning by degrees to feel 

 the effect of their mutual attraction, the pair quickly ac- 

 celerated their approach, and in the natural course of 

 events whirled once around their common center of 

 gravity and then escaped from each other along hyper- 

 bolic paths. This incident he supposes to have occurred 

 so long ago that the strange star has had sufficient time 

 to lose itself among the other stars near us. 



Coincidently with their thus drawing toward each 

 other, our author conceives the visitor as having raised 

 tides upon the sun (which may or may not have originally 

 possessed axial rotation) and thus stimulated the erup- 



