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



caught and returned, and hence the loss of heat by radiation from the 

 surface of the earth would have been reduced. 



2. If at the same time we suppose that the material now concen- 

 trated in the outer planets was dispersed in a broad nebulous or meteoric 

 belt mantling the heavens on the opposite side, another means would 

 be provided by which some portion of the heat radiated away would be 

 caught and returned to the earth, and a further small reduction in the 

 original receipt of heat from the sun may be made consistently with 

 the existence of life. This outer belt would be very tenuous, and its 

 effects correspondingly meager, but it is a factor to be considered in a 

 complete set of assumptions. 



3. If, in addition to this, we make the consistent assumption that 

 many other bodies of the heavens which are now concentrated into 

 suns or into dark bodies were then in a more dispersed nebulous or 

 meteoroidal condition, the general space of the stellar universe would 

 be partially mantled, and there would be less free scope for the escape 

 of the heat, solar and terrestrial alike, which is now freely lost through 

 the open regions of space. It ma}^ be conceived that there was a com- 

 mon blanketing of the heavens by the dispersal of its now concentrated 

 matter. This conception is the logical companion of the supposed 

 dispersal of the solar matter. If the volume of matter in the stellar 

 universe could be supposed to be sufficient, it might be so distributed 

 hypotheticall}^ as to mantle the whole heavens and largely prevent the 

 escape of central heat outward, just as the central heat of the more 

 concentrated bodies is conserved at the present time. Under this con- 

 ception the history of the stellar universe may be characterized as a 

 progressive clearing up of nebulosities and meteoroidal dispersions and 

 the concentration of its matter about certain points, leaving between 

 vast open spaces through which heat is now radiated away with a 

 facility unrealized in the earlier stages. The quantitative value of 

 such a suggestion must be left to the determination of astronomers, 

 who have the best data for forming a conjecture as to the ratio of 

 matter to space in the stellar universe and as to the possibilities of its 

 dispersion at a period coincident with the earlier stages of the earth's 

 histor3^ 



4. A modification of the conditions assumed in the foregoing para- 

 graphs may be postulated in which the earth is regarded as having 

 made its early growth within the primordial meteoric aggregate, per- 

 haps a great flattened meteoric spheroid, which initially extended 

 beyond Neptune in nebular fashion and whose present attenuated rep- 

 resentative may perhaps be found in the zodiacal light. In this case 

 the thermal environment of the early earth was that furnished by the 

 interior of the spheroid, though far out from the center. The condi- 

 tions only became external gradually as the growth of the planets 

 exhausted the peripheral portion of the meteoric spheroid. 



