438 The Doctrine of Evolution. 



be made in geology, it was necessary that physics and chem- 

 istry, the sciences which generalize the properties of matter, 

 in the mass and in the molecule, should be to some extent 

 apprehended; and it is almost startling to think how 

 modern all this is scarcely more than a hundred years 

 since Priestley discovered oxygen, since it became possible 

 to tell what goes on when you burn a log of wood on the 

 hearth ! and not so very much longer since Black discovered 

 latent heat and gave us a clew to what happens when water 

 freezes and melts or when it is turned into steam ! It is 

 only within fifty years that physics and chemistry have 

 begun to assume the form of coherent bodies of scientific 

 truth. Evidently geology could not be expected to take 

 scientific shape until late in the eighteenth century, or to 

 make any notable conquests before the nineteenth. But 

 when geology did win its first great triumph, about sixty 

 years ago, it was in some ways the most remarkable mo- 

 ment in the history of thought since the promulgation of 

 the Newtonian astronomy. Newton proved that the forces 

 which keep the planets in their orbits are not strange or 

 supernatural forces, but just such forces as we are familiar 

 with on this earth every moment of our lives. Geologists 

 before Lyell had been led to the conclusion that the general 

 aspect of the earth's surface with which we are familiar is 

 by no means its primitive or its permanent aspect, but that 

 there has been a succession of ages in which the relations of 

 land and water, of mountain and plain have varied to a 

 very considerable extent, in which soils and climates have 

 undergone most complicated vicissitudes, and in which the 

 earth's vegetable products and its animal populations have 

 again and again assumed new forms while the old forms 

 have passed away. In order to account for such wholesale 

 changes, geologists were at first disposed to imagine violent 

 catastrophes brought about by strange agencies agencies 

 which were perhaps not exactly supernatural, but in some 

 unspecified way different from the agencies that are now at 

 work in the visible and familiar order of Nature. But Lyell 

 proved that the very same kind of physical processes which 

 are now going on about us would suffice during a long period 

 of time to produce the changes in the inorganic world which 

 distinguish one geological period from another. Here, in 

 Lyell's geological investigations, there was for the first time 

 due attention paid to the immense importance of the pro- 

 longed and cumulative action of slight and unobtrusive 



