THE DOCTRINE OF EVOLUTION. 579 



things upon the earth's surface ,was once utterly different from 

 anything that human tradition can remember, and it was accord- 

 ingly quite natural that they should suppose that things have al- 

 ways been about as they are. The human mind can not transcend 

 experience. The man who has always lived in a comparatively 

 unchanged environment will, of course, never believe in a differ- 

 ent state of things until taught by some fresh experience. How 

 long it was before it was brought home to men that the testimony 

 of the unaided senses needs to be corrected by systematic observa- 

 tion and reasoning! From this point of view, as indeed from 

 some others also, the revolution in astronomical theory effected by 

 Copernicus was one of the greatest events in human history. Its 

 philosophic consequences were profound. In teaching men the ne- 

 cessity of going back of superficial appearances, and subjecting 

 their crude opinions to some kind of critical test, it was an object- 

 lesson of unsurpassed value. Along with this abrupt shifting of 

 man's apparent position in the universe, came the astonishing re- 

 sults of oceanic discovery, enlarging fourfold the dimensions of 

 the known world and bringing the mind into contact with organic 

 and inorganic nature in various new and unsuspected forms. Then 

 came the Newtonian astronomy, in which a generalization from 

 terrestrial physics was extended into the celestial spaces and 

 quantitatively verified. There was an immense enlargement of 

 the mental horizon, and the problems immediately connected with 

 it were enough to occupy the attention of all the foremost mathe- 

 matical minds for more than a century. It made man a denizen 

 of the solar system as well as of his own particular planet ; and in 

 these latter days, since the law of gravitation has been extended 

 to the sidereal heavens and spectrum analysis has begun to deal 

 with nebula?, there is abundant proof that properties of matter and 

 processes with which we are familiar on this earth are to be found 

 in some of the remotest bodies which the telescope can reach, and 

 it is thus forcibly impressed upon us that all are parts of one stu- 

 pendous whole. 



This enlargement of the mental horizon, from Newton to Kirch- 

 hoff, had reference to space. A similar enlargement with reference 

 to time was an indispensable preliminary to any correct under- 

 standing of how the world is made and what is going on in it. 

 But, before much headway could be made in geology, it was neces- 

 sary that physics and chemistry, 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 



