9 o POPULAR LECTURE* AND ADDRESSES. 



that transmutation has taken place through 

 " descent with modification by natural selection." 

 14. And now as to Professor Huxley's examina- 

 tion of my arguments. (I.) Referring to my estimate 

 of the retardation of the earth's rotational velocity 

 due to an imagined melting of ice from the polar 

 regions, he remarks that a certain accumulation of 

 polar ice since the mioccne epoch, and not more 

 than he imagines may in reality have taken place, 

 would produce five times as much acceleration 

 as the amount of the retardation which we have 

 estimated from the tides ; and he supposes that 

 this would " leave ! of a second per annum in the 

 way of acceleration." JUit the observed result is 

 retardation, and Professor Huxley's hypothesis as 

 to ice, if it were valid, would therefore prove re- 

 tardation by the tides six times as much as that 

 which we have ventured to estimate ! I am much 

 obliged to him for this suggestion, and also to Mr. 

 Croll for a suggestion which he has made to me 

 that the erosion of equatorial mountains and 

 deposition of detached matter at considerable dis- 

 tances from the equator, in either north or south 



