112 GEOLOGY. 



tive that the initial forms of life had been introduced long before, or 

 else that an evolution quite out of harmony with that which succeeded, 

 took place in the unknown interval antecedent to the record. Whence 

 the life was introduced is also quite unknown. The speculation that 

 it might have been brought to the earth from some other celestial 

 body by a fragment, in the form of a meteorite, is merely a refuge 

 from supposed geological, biological, and philosophical difficulties — 

 a merely temporary refuge in the face of prodigious improbabilities, 

 for it only throws back the problem of life-genesis without solving it. 

 There is nothing in known meteorites, save perhaps the existence 

 of hydrocarbons equally assignable to inorganic sources, to indicate 

 that they came from worlds with atmospheres and hydrospheres suited 

 to maintain such life as the problem presents. On the contrary, there 

 are the best of grounds for believing that meteorites came from bodies 

 in which the essential 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 characterized by quartz, orthoclase, the acid plagioclases, the 

 micas, the amphiboles, etc. ( Vol. I, p. 467), 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 accretion 

 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 not a tenable 

 refuge, and transfer from some other stellar system is the only obvious 

 recourse. 



Available time. — The accretion hypothesis affords an undetermined 

 lapse of time between the stage when conditions congenial to life were 

 first possible, 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 every presumption that the time was ample, and there is no occa- 

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



