DARWIN AND HIS CRITICS. 53 



the conditions of life upon a planet must needs 

 be regulated by the nature of its atmosphere 

 and the amount of its internal heat, as well 

 as by its distance from the sun, and the actual 

 amount of imponderables received from this 

 source. In the same way, it is inconsequential 

 to argue that life cannot exist in planets 

 nearer or further from the sun than our own ; 

 for it is quite conceivable that each planet 

 may absorb a sufficient amount of imponder- 

 ables from the sun for its own necessities, 

 whatever its distance. Such an argument could 

 only hold good if the planets were all of 

 similar constitution to the earth ; but although 

 probably composed of the same materials, 

 we know that they differ in size, density, 

 and probably in the composition of their 

 atmospheres. 



Sir J. Lubbock* argues, that while geological 

 data are still very doubtful, they indicate a very 

 much greater antiquity for the earth than that 

 assigned to it by Thomson. The denudation 

 of the Weald alone is estimated to have re- 

 quired more than 150,000,000 years. Wallace 



* " Prehistoric Times," ch. xii. 



