88 HUXLEY 



question of the age of the earth as determining the 

 length of time which elapsed before it became cool 

 enough to be the abode of life. On this question the 

 physicists and the biologists were at issue. 



At the outset the earth was a mass of glowing, in- 

 candescent gas, hurled-off, like its fellow-planets, 

 from the vast nebula which was to condense into the 

 solar system. Passing, under the continuous loss of 

 heat, from the gaseous through the liquid and viscous 

 to the solid state, it reached a degree of temperature 

 which permitted of the existence of life upon its sur- 

 face. The first living things were plants, and the 

 carbon, which is an essential element in their struc- 

 ture, could not be detached from the atmosphere 

 except at a temperature u somewhat above the freez- 

 ing-point, and somewhat less than half-way to the 

 boiling-point of water." Hence the question, At 

 what period of its history did our globe, or that por- 

 tion of it on which life first appeared (probably, as 

 BufFon suggested, the polar area, as this would be the 

 earliest to cool), arrive at that temperature ? To this 

 the mathematical physicists, at the head of whom 

 stands Lord Kelvin, essayed answer, the data for 

 which were supplied — I, by the rate at which the 

 earth parted with its store of heat ; 2, by the decrease 

 in the length of its day ; and 3, by the time that the 

 sun, as the source of life, has illuminated the earth. 



