THE AGE OF THE EARTH 71 



the same deposition occurring all over the globe. Where 

 one kind of rock was formed for a very long period in one 

 place, so that it attained to great thickness, the next succeed- 

 ing stratum may in that particular place have been very thin. 



And again, as pointed out by Mr. Wallace,* although the 

 denudation of the land by the agency of rain extends over 

 very large areas, the rivers deposit all the silt which they 

 carry down to the sea within 150 miles from the coast, and 

 even this limit is reached only opposite to the mouths of large 

 rivers. It is, therefore, necessary for the purpose of calcula- 

 tion that an estimate should be made of the average 

 thickness of the sedimentary rocks all over the globe, beneath 

 the bottom of the oceans as well as over existing continents. 

 Until this has been done, and at present it seems to be an 

 impossible task, the geological figures are of comparatively- 

 little value. 



To biologists this controversy is of great interest, although 

 they cannot be said to have any claim to an independent 

 opinion, since they have absolutely no standard by which to 

 gauge evolutionary time. Although plants and animals have 

 been changed profoundly by cultivation and breeding within 

 historic times, there is no evidence that they have changed 

 within the historic period without Man's interference. It is 

 impossible to prove that the hands of the evolutionary clock 

 have moved. Such negative evidence is of value, however, 

 as showing that if evolution proceeds so slowly that it cannot 

 be detected in the process, even though its records extend 

 over several thousands of years, it must have required a long 

 period to allow of the changes in the forms of living things 

 which are pictured in the fossil-bearing rocks. When 

 Charles Darwin was submitting to the world his doctrine of 

 the Origin of Species, he felt it necessary to insist, "how 

 incomprehensively vast have been the past periods of time," 

 because he foresaw that the objection would inevitably be. 



* " Island Life," chap. x. 



