394 HEREDITY AND ENVIRONMENT 



tions, it is certain that changes take place in 

 the inheritance factors. Sometimes factors 

 drop out, as in the white sweet peas shown in 

 Fig. 57 and in many other cultivated races 

 of plants and animals, thus producing regres- 

 sive mutations. Indeed most of our domestic 

 animals and cultivated plants have arisen by 

 the omission of something which their wild 

 ancestors had. Occasionally new factors are 

 added thus producing progressive mutations, 

 which are of special importance in evolution. 



It is often said that evolution from the 

 amoeba to man necessarily involves the addi- 

 tion of many new inheritance factors. This is 

 probably true, but the addition of new fac- 

 tors does not mean their creation. New hered- 

 itary factors are to be thought of as we think 

 of new chemical compounds, which are formed 

 by new combinations of the same old elements, 

 or as we think of new elements, such as helium 

 and radium emanation, which are formed by 

 dissociation of radium. As compared with 

 chemical elements the factors of heredity are 

 probably very complex things and the new fac- 

 tors which appear in the course of evolution 



