154 THE GENESIS OF SPECIES. [CHAP. 



planet ; and 3, The temperature of the interior of the 

 earth. The result arrived at by these investigations is a 

 conclusion that the existing state of things on the earth, 

 life on the earth, all geological history showing continuity 

 of life, must be limited within some such period of past 

 time as one hundred million years. The first question 

 which suggests itself, supposing Sir W. Thomson's views to 

 be correct, is : Has this period been anything like enough 

 for the evolution of all organic forms by " Natural Selec- 

 tion " ? The second is : Has this period been anything like 

 enough for the deposition of the strata which must have 

 been deposited if all organic forms have been evolved by 

 minute steps, according to the Darwinian theory ? 



As to Sir William Thomson's views the author of this 

 book cannot presume to advance any opinion ; but the 

 fact that they have not been refuted pleads strongly in 

 their favour, when we consider how much they tell 

 against the theory of Mr. Darwin. The last-named author 

 only remarks that "many of the elements in the calcu- 

 lation are more or less doubtful," 1 and Professor Huxley 2 

 does not attempt to refute Sir William Thomson's 

 arguments, but only to show cause for suspense of judg- 

 ment, inasmuch as the facts may be capable of other 

 explanations. 



Mr. Wallace, on the other hand, 3 seems more disposed 

 to accept them, and, after considering Sir William's 

 objections and those of Mr. Croll, puts the probable date 

 of the beginning of the Cambrian deposits 4 at only 



1 "Origin of Species," 5th edition, p. 354. 



2 See his address to the Geological Society, on February 19, 1869. 



3 See Nature, vol. i. p. 399, February 17, 1870. 



4 Ibid. vol. i. p. 454. 



