6 THE AGE OF THE EARTH 



and that during these vast yet quite unknown periods of 

 time the world swarmed with living creatures.' 



The depth of his conviction in the validity of this con- 

 clusion is seen in the fact that the passage remains sub- 

 stantially the same in later editions, in which, however, 

 Cambrian is substituted for Silurian, while the words 

 • yet quite unknown ' are omitted, as a concession, no 

 doubt, to Lord Kelvin's calculations, which he then pro- 

 ceeds to discuss, admitting as possible a more rapid 

 change in organic life, induced by more violent physical 

 changes. 1 



We know, however, that such concessions troubled 

 Darwin much, and that he was really giving up what his 

 judgement still approved. Thus he wrote to Wallace on 

 April 14, 1869 : ' Thomson's views of the recent age of 

 the world have been for some time one of my sorest 

 troubles. . . .' And again, on July 12, 1871, alluding to 

 Mivart's criticisms, he says : ' I can say nothing more 

 about missing links than what I have said. I should 

 rely much on pre-Silurian times ; but then comes Sir W. 

 Thomson, like an odious spectre.' 



Huxley's demands for time in order to account for 

 pre-Cambrian evolution, as he conceived it, were far 

 more extensive. Although in 1869 he bade the naturalist 

 stand aside and take no part in the controversy, he had 

 nevertheless spoken as a naturalist in 1862, when, at the 

 close of another Anniversary Address to the same 

 Society, he argued from the prevalence of persistent 

 types ' that any admissible hypothesis of progressive 

 modification must be compatible with persistence without 

 progression through indefinite periods ' ; and then main- 

 tained that ' should such an hypothesis eventually be 

 proved to be true . . . the conclusion will inevitably 

 present itself that the Palaeozoic, Mesozoic, and Cainozoic 

 faunae and florae, taken together, bear somewhat the same 

 proportion to the whole series of living beings which 

 have occupied this globe as the existing fauna and flora 

 do to them '. 



Herbert Spencer, in his article on Illogical Geology 

 1 Sixth ed. 1872, p. 286. 



