100 HEREDITARY CHARACTERS 



the age of the earth, and the argument that the available 

 time was insufficient to allow for evolution by natural selec- 

 tion was upheld by many. Lord Kelvin and Sir George 

 Darwin were considered liberal in allowing five hundred 

 million years as the age of the sun. Other arguments, 

 limiting the available time, were based upon the heat con- 

 ductivity of the earth. The discovery of radium has, how- 

 ever, modified the time allowance to such an extent that 

 " we are now in a position to say that the physicist can make 

 no calculation either as to the probable or possible age of life 

 on the earth." * 



We are then driven to the conclusion that whatever the 

 results of mutations may be, they are not usually the ground- 

 work of evolution. It is quite possible that occasionally a 

 favourable mutation may be preserved, but according to the 

 whole evidence at present available, it seems probable that 

 under natural conditions this could happen but rarely, and 

 that the usual course of events is that mutations are elimi- 

 nated and adaptation depends upon the selection of small 

 and continual variations. But adaptation is, in a sense, 

 evolution ; so it seems equally certain, according to our 

 present state of knowledge, that evolution has also depended 

 upon small variations, mutations having played little or no 

 part in the phenomenon. It also appears that the way in 

 which new characters are produced and preserved in domes- 

 ticated races is very different from what happens in races 

 living under natural conditions. Man is able to select large 

 variations (mutations, sports), and to preserve them. He is 

 able to do away with the necessity for accurate adaptation 

 in the organisms under his protection. As he frequently 

 if not generally, selects large variations (mutations), it is 

 certain that he has unknowingly selected the character of 

 " sporting " or " mutating " in most domesticated races. The 

 property of variation is an hereditary character just as 

 subject to selection as any other. Under natural conditions 



1 Professor Perry writing to Professor E B. Poulton, quoted by the latter 

 in Essays on Evolution. 



