HI LECTURES ON EVOLUTION 135 



his time from the geologist. The geologist, con- 

 sidering the rate at which deposits are formed 

 and the rate at which denudation goes on upon 

 the surface of the earth, arrives at more or less 

 justifiable conclusions as to the time which is 

 required for the deposit of a certain thickness 

 of rocks; and if he tells me that the Tertiary 

 formations required 500,000,000 years for their 

 deposit, I suppose he has good ground for what 

 he says, and I take that as a measure of the 

 duration of the evolution of the horse from the 

 Orohippus up to its present condition. And, if 

 he is right, undoubtedly evolution is a very slow 

 process, and requires a great deal of time. But 

 suppose, now, that an astronomer or a physicist 

 for instance, my friend Sir William Thomson 

 tells me that my geological authority is quite 

 wrong ; and that he has weighty evidence to 

 show that life could not possibly have existed 

 upon the surface of the earth 500,000,000 years 

 ago, because the earth would have then been 

 too hot to allow of life, my reply is : " That is 

 not my affair; settle that with the geologist, 

 and when you have come to an agreement among 

 yourselves I will adopt your conclusion." We 

 take our time from the geologists and physicists ; 

 and it is monstrous that, having taken our time 

 from the physical philosopher's clock, the phy- 

 sical philosopher should turn round upon us, and 

 say we are too fast or too slow. What we 



