DARWIN AND THE ORIGIN OF SPECIES 241 



of change according to circumstances. The greater the dif- 

 ference of the temperature produced between the surrounding 

 objects, the greater the physical changes they will effect, but the 

 degradation is in all cases inevitable. Finally, the sun's rays 

 take the form of heat, whether they raise water or vegetation, or 

 do any other work, and in this form the energy quits the earth 

 radiated into distant space. Nor would this gradual degrada- 

 tion be altered if space were bounded and the planets inclosed 

 in a perfect non-conducting sphere. Everything inside that 

 sphere would gradually become equally hot, and when this con- 

 summation was reached no further change would be possible. 

 We might say (only we should not be alive) that the total 

 energy of the system was the same as before, but practically the 

 universe would contain mere changeless death, and to this con- 

 dition the material universe tends, for the conclusion is not 

 altered even by an unlimited extension of space. Moreover, the 

 rate at which the planetary system is thus dying is perfectly 

 mensurable, if not yet perfectly measured. An estimate of the 

 total loss of heat from the sun is an estimate of the rate at which 

 he is approaching the condition of surrounding space, after 

 reaching which he will radiate no more. We intercept a few of 

 his rays, and can measure the rate of his radiation very accu- 

 rately : we know that his mass contains many of the materials 

 our earth is formed of, and we know the capacity for heat and 

 other forms of energy which those materials are capable of, and 

 so can estimate the total possible energy contained in the sun's 

 mass. Knowing thus approximately, how much he has, and 

 how fast he is losing it, we can, or Professor Thomson can, cal- 

 culate how long it will be before he will cool down to any given 

 temperature. Nor is it possible to assume that, per contra, he 

 is receiving energy to an unlimited extent in other ways. He 

 may be supplied with heat and fuel by absorbing certain planetary 

 bodies, but the supply is limited, and the limit is known and 

 taken into account in the calculation, and we are assured that 

 the sun will be too cold for our or Darwin's purposes before 

 many millions of years a long time, but far enough from count- 

 less ages ; quite similarly past countless ages are inconceivable, 

 inasmuch as the heat required by the sun to have allowed him 

 to cool from time immemorial, would be such as to turn him 

 VOL. I. R 



