The Doctrine of Evolution. 437 



Until geology had made some headway, men had no means 

 of knowing that the state of things upon the earth's surface 

 was once utterly different from anything that human tradi- 

 tion can remember, and it was accordingly quite natural 

 that they should suppose that things have always been about 

 as they are. The human mind can not transcend experi- 

 ence. The man who has always lived in a comparatively 

 unchanged environment will, of course, never believe in a 

 different 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 observation 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 conse- 

 quences were profound. In teaching men the necessity 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 shift- 

 ing of man's apparent position in the universe, came the 

 astonishing results 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 

 mathematical minds for more than a century. It made 

 man a denizen of the solar system as well as of his own par- 

 ticular 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 nebulas, 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 

 stupendous whole. 



This enlargement of the mental horizon, from Newton to 

 Kirchhoff, had reference to space. A similar enlargement 

 with reference to time was an indispensable preliminary to 

 any correct understanding of how the world is made and 

 what is going on in it. But, before much headway could 



