EXCURSIONS OF AN EVOLUTIONIST 



Ham Thomson's conclusion contains more or 

 less that is hypothetical, it is well enough to 

 adopt it provisionally ; and I shall do so here. 

 Of the ten aeons, then, into which I have sup- 

 posed geological time to be divided, we will 

 suppose that each is about ten million years in 

 duration ; bearing in mind that, while it is 

 highly improbable that the lapse of time has 

 been very much less than this, it may not im- 

 probably have been considerably greater. Ac- 

 cording to this, the minimum antiquity for the 

 beginning of the Eocene period would be about 

 five million years. 



If these periods seem short in comparison 

 with the enormous quantity of work that has 

 been done, both in the tearing down and re- 

 building of the earth's crust and in the modifi- 

 cation of the forms of animals and vegetables, 

 it is no doubt largely due as both Mr. Dar- 

 win and Mr. Croll have reminded us to the 

 fact that it is almost impossible for us to frame 

 an adequate conception of what is meant by a 

 million years. We are wont to use these great 

 arithmetical figures glibly, and without compre- 

 hending their import. Mr. Croll has done 

 something to help us in this matter. " Here is 

 one way," he says, " of conveying to the mind 

 some idea of what a million of years really is. 

 Take a narrow strip of paper, an inch broad or 



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