EXCURSIONS OF AN EVOLUTIONIST 



mals dominant in the animal world, could have 

 begun to assume anything even distantly re- 

 sembling the aspect under which we know it. 

 Yet if we could be suddenly taken back, and 

 permitted to inspect a landscape of the earliest 

 Tertiary epoch, we should probably be far 

 more forcibly struck with the differences than 

 with the points of resemblance. 



In this succinct view I have supposed the 

 whole of the life history of our planet to be 

 arbitrarily divided into ten equal periods. What, 

 it may be asked, is supposed to have been the 

 actual duration of one of these aeons ? I am 

 well aware that to such a question no definite 

 answer can be given. The geologist deals only 

 with relative, not with absolute, quantities of 

 time : he can only say that the time has been 

 long enough for a certain enormous amount 

 of work to be performed, but he has nothing 

 with which to measure its duration in years. 

 Nevertheless, while fully admitting all this, 

 one may perhaps venture to give a provisional 

 answer for a provisional purpose. For the 

 present it will be enough to recall Sir Wil- 

 liam Thomson's ingenious speculations as to 

 the limits of the antiquity of life upon the 

 earth. Reasoning from the sources of the sun's 

 heat, and from the length of time which it 

 would take a body like the earth to cool so as 

 to produce the present increment of tempera- 

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