Mutation 205 



variation is a mutation. The word so used implies nothing 

 as to extent or nature of the variations that are herit- 

 able; it is a name for all that occur. 



But in many minds the term mutation means more than 

 this. Many new heritable characters in higher organisms 

 are found to be, when the character has reached its com- 

 plete development in the adult, changes of considerable ex- 

 tent. They differ from the original condition by a large 

 step ; they affect many parts of the organism ; or profoundly 

 change particular organs. Such were most of the hereditary 

 changes found by de Vries (1901) in the evening primroses, 

 CEnothera ; from this work arises the general use of the word 

 mutation for hereditary changes. Such too are many of the 

 hereditary changes observed in the fruit-fly, Drosophila, by 

 Morgan and his associates. Thus, in the typical individuals 

 the eyes are red ; these sometimes produce offspring in which 

 the eyes are white, and this mutation is inherited. The 

 hereditary change has come, not by minute changes of shade, 

 gradually altering from generation to generation ; but by a 

 complete change in one generation from red to white. 



From such cases, the word mutation has come to mean in 

 the minds of many persons an extensive change; a sudden 

 jump from one condition to another; a "saltation." And 

 the statement that evolutionary changes occur by mutation 

 has come to mean that they do not take place in gradations ; 

 in minute, almost imperceptible alterations from generation 

 to generation, but always by large leaps. Possibly this is 

 the usual idea of what is meant by the mutation theory of 

 evolution. 



Such slight changes as we have described in the preceding 

 lectures as occurring in Difflugia and other Protozoa do not 

 agree with this idea. Is there a contrast in this respect 

 between what occurs in the higher and the lower organisms? 



