4 PLANT-BREEDING 



and may accumulate, in the lapse of time, to any degree. 

 All of the characters of living organisms were simply assumed 

 to be due to this slow process of gradual evolution guided 

 by natural selection. 



Here, however, a first difficulty arose. We do not ob- 

 serve actual specific changes in nature. To meet this ob- 

 jection Darwin assumed the changes to be so slow as to be 

 invisible to us. Even the life time of a man would not be 

 sufficient to control them. By this supposition the evolution 

 of a flower or a seed or of highly differentiated organs (such 

 as the leaves of insectivorous plants) would require an enor- 

 mous time. From this a calculation could be made as to 

 the time required for the whole range of evolution of the 

 vegetable and animal kingdoms. The result was that many 

 thousands of millions of years were considered to be the 

 smallest amount that would account for the development 

 of life on earth from the very first beginning until the appear- 

 ance of mankind. 



Physicists and astronomers have objected to this con- 

 clusion. The objection has been brought forward from the 

 time when Darwin published his calculation. It has never 

 relented and has often threatened to impair the whole theory 

 of descent. The results of physical and astronomical cal- 

 culations concerning the age of life on this earth differ so 

 widely from the demands made by the theory of slow evolu- 

 tion as to be considered incompatible with them. The de- 

 ductions made by Lord Kelvin and others, from the central 

 heat of the earth, from the rate of the production of the cal- 

 careous deposits, from the increase of the amount of salt 

 in the water of the seas, and from various other sources, 

 indicate an age for the inhabitable surface of the earth of 

 between twenty and forty millions of years only. This 

 large discrepancy has always been a weapon in the hands of 

 the opponents of the evolutionary idea, and there can be no 



