26 EVOLUTION 



for all the phenomena which the crust of the earth 

 exhibits. It is now more generally supposed that in 

 very early times forces similar in kind to those in 

 action at the present day may have exhibited con- 

 siderably greater violence. 



To produce the present condition of the surface of 

 the earth by the action, gradually accumulated, of 

 such processes of denudation and upheaval as are now 

 going on around us, vast periods of time are clearly 

 necessary. The early evolutionists, having once got 

 rid of the idea that the date given by Bishop Usher 

 as that of the creation of the world is a necessary and 

 integral part of religion, immediately allowed their 

 imaginations to run riot with regard to the amount 

 of time at their disposal. Since this question of the 

 extent of geological time has an important bearing on 

 the problem of organic as well as upon that of inorganic 

 evolution, it will be well to pay some attention to 

 more recent views upon the subject. 



Some years ago the generous ideas of biologists as 

 well as of geologists were to a great extent shattered 

 by the calculations of Lord Kelvin. These were based 

 upon three separate sets of data, which we may enume- 

 rate without entering into a lengthy explanation of 

 the calculations involved. The evidence made use of 

 consisted of (i) the rate of the earth's rotation, as 

 affected by tidal retardation ; (2) the rate of secular 

 cooling of the earth, as deduced from the rate at which 

 the temperature of the earth's crust rises on passing 

 inward from the surface ; and (3) the rate of cooling of 

 the sun by radiation. The three calculations were 



