Uecembeb 31, 1000] 



SCIENCE 



941 



Weakness also arose in another quarter. 

 One of the main props of the gaseous or 

 quasi-gaseous hypotheses was, as already 

 remarked, the general conviction based on 

 dynamical grounds that condensation from 

 any other nebulous state than gaseous or 

 quasi-gaseous would give revolutions and 

 rotations to the planetary system at vari- 

 ance with those actually possessed. A re- 

 examination, however, near the close of the 

 century, developed grounds for the convic- 

 tion that a gradual gathering in of matter 

 from a scattered orbital .state would give 

 rotations and revolutions quite as well in 

 accord with the facts formerly known and 

 seemingly even better in accord with new 

 facts recently brought to light. 



Thus toward the close of the last century 

 there arose from different quarters cogent 

 reasons for a reconsideration of the pre- 

 vailing general view, and with it a recast 

 of the former forecast. Further scrutiny 

 added new doubts to those that had pre- 

 viously arisen and in the end the verity of 

 the older hj'potheses of genesis was chal- 

 lenged and new conceptions based on or- 

 bital dynamics, in contrast to gaseous dy- 

 namics, were offered in their stead. 



It is not appropriate for me to say that 

 this challenge was .successful, or that the 

 older conceptions of the earth's origin are 

 to be laid on the shelf. As an advocate of 

 the method of multiple working hypotheses 

 it belongs to me to beg of you to save and 

 to use, so far as you can find use in them, 

 all the hypotheses that seem to you to be 

 capable of working at all. Much less 

 would it be appropriate for me to affirm 

 that any form of the newer conceptions is 

 entitled to take the place of the older in 

 your complete confidence. The final adju- 

 dication of genetic hypotheses can only 

 come of long and patient trial by searching 

 analysLs, by scrutinizing logic and by ap- 

 plication to the multitudinous phenomena 



which not only the earth, but the solar and 

 stellar systems present. It is sufficient 

 warrant for the present review, however, 

 that not a few of the more incisive students 

 of these things have been led to seriously 

 reconsider the foundations of the hypoth- 

 eses of earth-genesis that have been offered, 

 old and new, and to examine with renewed 

 care the interpretations and inferences that 

 have been made to hang upon them. 

 Whatever may be your personal leanings 

 you will no doubt agree that it seems less 

 laudable now to hang prophecies of the 

 future upon hypothes&s of genesis, than 

 when certain of these hypotheses received 

 the almost universal assent of those then 

 best qualified to hold opinions respecting 

 them. 



It does not seem to be going too far, 

 moreover, to say that whereas we seemed 

 to be shut up to hypotheses of genesis that 

 gave the earth a gaseo-molten state at the 

 start, it now seems, to some .students at 

 least, possible that the earth inherited a 

 quite different state from a slow growth 

 from planetesimal or other accretions. If 

 diver.se views are thus permissible they 

 offer alternative working conceptions and 

 thus help to give freedom of interpretation 

 while they .s-timulate observation on the 

 critical phenomena. We may therefore be 

 permitted first to review the .states as.signed 

 the early earth by the competitive genesis 

 offered and then the critical phenomena 

 that bear upon the earth's future. 



Quite in contrast with the older pictures 

 of a primitive earth cooling from a gaseous 

 state, the planetesimal hypothesis, which 

 may be taken as representative of theories 

 based on concentration from a dispersed 

 orbital state, postulates a solid earth grow- 

 ing up slowly by accessions and becoming 

 clothed gradually with an atmosphere and 

 a hydrosphere. Each of the fundamental 

 parts, the earth, the air and the water, is 



