710 Mutations 



is quite sufficient to enable us to discern the 

 laws and to conjecture the outlines of the whole 

 scheme of evolution. 



A grave objection which has often, and 

 from the very outset, been urged against Dar- 

 win's conception of very slow and nearly im- 

 perceptible changes, is the enormously long time 

 required. If evolution does not proceed any 

 faster than what we can see at present, and if 

 the process must be assumed to have gone on 

 in the same slow manner always, thousands of 

 millions of years would have been needed to de- 

 velop the higher types of animals and plants 

 from their earliest ancestors. 



Now it is not at all probable that the duration 

 of life on earth includes such an incredibly 

 long time. Quite on the contrary the lifetime 

 of the earth seems to be limited to a few 

 millions of years. The researches of Lord Kel- 

 vin and other eminent physicists seem to leave 

 no doubt on this point. Of course all esti- 

 mates of this kind are only vague and approx- 

 imate, but for our present purposes they may 

 be considered as sufficiently exact. 



In a paper published in 1862 Sir William 

 Thomson (now Lord Kelvin) first endeavored to 

 show that great limitation had to be put upon 

 the enormous demand for time made bv Lvell, 

 Darwin and other biologists. From a consider- 



