376 ANNUAL KEPORT SMITHSONIAN INSTITUTION, 1910. 



3'onr complete confidence. The final adjudication of a genetic hypo- 

 thesis of such a remote and complex an event as the birth of the 

 earth and the solar system can onljT^ come of protracted scrutiny by 

 means of diverse forms of searching analyses and trenchant logic, 

 and of long and patient trial in testing the hypothesis by the multi- 

 tudinous phenomena constantly coming to light from the earth, the 

 solar system, and the heavens beyond. 



It is sufficient warrant for the present review, however, that not a 

 few incisive students of celestial dynamics have been led to seriously 

 reconsider the foundations of the hypotheses of earth genesis, and 

 that not a few geologists have been led to scrutinize with renewed 

 care the inferences and interpretations that have been hung upon 

 past theories of earth genesis. Whatever may be your personal 

 leanings, you will no doubt agree that it seems less permissible now 

 to hang prophesies of the future upon challenged hypotheses of 

 genesis than it might once have been, Avhen a certain hypothesis or a 

 certain class of hypotheses received the almost universal assent of 

 those' who seemed then best qualified to hold opinions respecting 

 them. 



It does not seem to be going too far to say that whereas we for- 

 merly seemed shut up to hypotheses of genesis that assigned the 

 earth a gaseo-molten state at the start, it now seems to some of us 

 at least that the earth may have inherited a quite different state from 

 a slow growth by the ingathering of small bodies of a planetesimal 

 nature. If views that are thus fundamentally diverse are permissible, 

 and if these give rise to a wide range of alternative working concep- 

 tions, we are freed from some of the constraints of interpretation 

 that have hampered our reading of past history and colored our out- 

 look upon the future. Let us, therefore, pass in brief review the 

 states assigned the early earth by the newer conception of earth 

 genesis that we may gain a concrete impression of the lines of inter- 

 pretation it opens to us, and then let us turn to the critical phenomena 

 of the actual record as the more solid basis for a forecast of the 

 planet's future. 



Quite in contrast with the older pictures of the primitive earth, 

 the planetesimal hypothesis — and this is entitled to be taken as the 

 type of theories based on concentration from a scattered orbital 

 state — postulates a solid earth groAving up slowly by accessions and 

 coming to be clothed gradually with an atmosphere and hydrosphere. 

 The earth, the air, and the water are made to grow up together from 

 smaller to larger volumes without necessarily attaining a very high 

 temperature. The sources that at the first had furnished the body 

 of the ocean and the air, though they fell off as time went on, still 

 continued to serve as means of replenishment, and to act as an offset 

 to the familiar agencies of loss far down into the later ages, if indeed 



