154 THE GENESIS OF SPECIES [Chap. 



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

 earth. The result arrived at hy 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 ^vithin some such period of past 

 time as one huneh'cd 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 tlie evolution of all organic forms by " Xatural Selec- 

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

 enough f<jr the deposition of the strata Avliich must have 

 been deposited if all organic forms have been evolved by 

 mimUe steps, according to the Darwinian theory ? 



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

 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," ^ and Professor Huxley- 

 does not attempt to refute Sir William Thomson's 

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

 ment, inasmuch as the facts mai/ he capable of other 

 explanations. 



Mr. Wallace, on the other hand,^* 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^ at only 



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



2 See his atkircss to the (noh)gii'al Society, on February 19, 1869. 



' See Nature^ vel. i. j). 399, Fehruiuy 17, 1870, 



* Ibid. vol. i. p. 454. 



