THE AGE OF THE EARTH AS AN ABODE FITTED FOR LIFE. 243 



5. The foregoing hypotheses, which do not seem to be so completely 

 out of accord with the possibilities of the case as to be inadmissible 

 tentatively in the absence of a positive solution of the earl}- terrestrial 

 environment, are concerned with the external relations of the earth. 

 If we turn to the earth itself it may be remarked that the nature of 

 its atmosphere ver}^ radically conditions the- amount of heat requisite 

 for the support of life. Dr. Arrhenius has recently made an elaborate 

 computation relative to the thermal influence of certain factors of the 

 atmosphere, and has arrived at the conclusion that an increase of the 

 atmospheric carbon dioxide to the amount of three or four times the 

 present content would induce such a mild climate in the polar regions 

 that magnolias might again flourish there as they did in Tertiary times. 

 On the other hand, he concluded that a reduction of less than 50 per 

 cent would induce conditions analogous to those of the glacial period 

 of Pleistocene times. The vast quantities of carbon dioxide repre- 

 sented in the carbonates and carbonaceous deposits of the earth's crust 

 imply great possibilities of change in the constitution of the atmos- 

 phere of the earth in respect to this most critical element. 



6. But there are more radical considerations that relate to the early 

 thermal history of the earth. To be sure, if we are forced to adopt 

 the hypothesis of a white-hot liquid earth, with all its extravagant 

 expenditures of energy in the early youth of the earth, we can take no 

 advantage of these possible resources, but under the supposition that 

 the meteorites gathered in with measurable deliberation it is theoret- 

 ically possible to find conditions for a long maintenance of life on the 

 earth, with little or no regard to the amount of heat which the early 

 sun sent to it. In the earliest stages of the aggregation of the earth 

 under this hj^pothesis, while it was yet small, it can scarcely be sup- 

 posed to have been habitable, because its mass was not suflEicient to 

 control the requisite atmospheric gases; but when it had grown to the 

 size of Mars — that is, to a size representing about one-tenth of its pres- 

 ent aggregation, or, to be safe, when it had grown to twice the size of 

 Mars, or about one-fifth of its present mass — it would have been able 

 to control the atmospheric gases and water, and, so far as these essential 

 items are concerned, it would have presented conditions fitted for the 

 presence of life. At this stage the larger portion, four-fifths by 

 assumption, of the matter of the earth would yet be in the meteoroidal 

 form and doubtless more or less closely associated with the growing 

 nucleus. If the infalling of this four-fifths of the material of the earth 

 were duly timed, so as to be neither too fast nor too slow, it would 

 give hj its impact upon the atmosphere of the earth a sufficiency' both 

 of heat and of light to maintain life upon the surface of the earth. 

 The plunging down of these meteorites upon the surface might be more 

 or less destructive to the life, but only proportionately more so than 

 the fall of meteorites to-da}'. It would not be necessarily fatal to life, 



