400 NATURE AND MAN. 



to astronomers. Of course, it may be said that the Creator, when 

 he set the Moon in the firmamant, ordained that she should for 

 ever turn the same face to the Earth. But no man of scientific 

 habits of thought could rest satisfied with such a notion. The 

 probabilities were many millions to one in favour of some physical 

 cause for so singular an eff'ect; and such a cause has recently 

 been discovered by Helmholtz, who has shown that the continuous 

 retardation produced by ancient tides would at last bring the 

 moon into the only attitude it could permanently retain without 

 being subjected to further incessant disturbance. 



One more important evidentiary fact I have still to adduce, 

 which forms the connecting hnk between astronomical and geo- 

 logical evolution, and brings what may be now designated as the 

 scientific certainties of the past history of our own globe, to bear 

 on the history of every other body in the universe. I refer to the 

 determination of the high ititernal temperature of the Earth, v.'hich 

 now rests upon so wide a basis of concurrent observations, that 

 no one capable of scientifically appreciating their value any longer 

 entertains the smallest doubt as to the fact. And this fact can 

 only be rationally accounted for, as the result of gradual cooling 

 of the entire mass from a temperature higher than that now pos- 

 sessed by its hottest interior, by the radiation of heat from its 

 surface. For, as Sir William Thomson has tersely remarked, " If 

 " we were to find a hot stone in a field, we could say with entire 

 " certainty that this stone had been in the fire, or some other hot 

 "place, within a limited period of time." 



Astronomical Evolution, then, lands us in the idea of a globe 

 of molten matter, over whose surface a crust is beginning to form ; 

 and it is at this point that geology takes up the inquiry, and aims 

 to give a consistent history of the long succession of changes 

 which that crust has since undergone — in other words, to trace 

 the "Evolution" of its existing from its primitive condition. 

 Here, again, two distinct lines of inquiry may be pursued. One 

 of these, leading us onward in time from the assumed beginning, 

 furnishes us with those great dynamical conceptions, that help us 

 to account alike for the vast movements whose evidence we trace 

 in the elevation of continents and of mountain-chains, and for the 

 local developments of heat which have shown themselves in 



