FUNDAMENTAL PROBLEMS OF GEOLOGY. 249 



problem presents. On the contrarj', there is the l^est of grounds 

 for believing that meteorites came from bodies in which the essen- 

 tial conditions of life were wanting; for, besides the absence of free 

 oxygen and water, there is an absence of the products assignable to 

 weathering and to those rock changes that spring from the presence 

 of an atmosphere and hydrosphere. These embrace a large portion 

 of all known rocks in the outer part of the earth, such as are char- 

 acterized by quartz, orthoclase. the acid plagioclases, the micas, the 

 amphiboles, as well as the sedimentary rocks. The absence of these 

 in the meteorites is peculiarly significant because of their abundance 

 in the earth. The hypothesis of the foreign importation of life 

 encounters a special difficulty under the planetesimal hypothesis, in 

 that the planets were all forming at the same time. Under the other 

 hypotheses the outer planets may have formed earlier than the 

 inner ones, and an earlier evolution of life may have taken place in 

 one of the older planets, whence a transference to the earth is barely 

 conceivable. Under the accretion hypothesis even this is scarcely a 

 tenable refuge, and transfer from some other stellar system is the 

 only obvious recourse. 



The planetesimal hypothesis affords an undetermined lapse of time 

 between the stage when conditions congenial to life were first possi- 

 ble and the stage when the first fairly legible record was made in the 

 Cambrian period. To this unmeasured period the whole pre-record 

 evolution of life, whatever be its method, may be referred, with a strong 

 presumption that the time was ample and that there is no occasion 

 for an evasion of the profound problem of life genesis by referring 

 it to some distant and unknown body ; nor is the problem vexed by 

 duress of severe time limits. A theoretical scantiness of time for a 

 prolonged evolution previous to the Cambrian period has been deduced 

 from a molten earth, but this does not apply to the planetesimal 

 hypothesis. The supposed limitation of the sun's thermal endurance 

 would apply if the arguments could be trusted, but their foundation 

 has been cut away by recent discoveries. It is not the least of the 

 virtues of the planetesimal hypothesis that it opens the way to a 

 study of the problem of the genesis and early evolution of life free 

 from the duress of excessive time limits and of other theoretical ham- 

 perings, and leaves the solution to be sought untrammeled, except 

 by the conditions inherent in the problem itself, which are surely 

 grave enough. 



It is assumed that the conditions on which life is now dependent 

 were prerequisites to its introduction. As already indicated, an 



