72 GENETICS 



however, may be more suitably considered later after 

 what is meant by a "Mendelian recessive" has been 

 made clear. 



It is extremely doubtful, however, whether any 

 recombination of parental characters in a hybrid may 

 properly be called a mutation, since no character 

 strictly new is thus produced. 



10. A Summary of the Mutation Theory 



• The main features of the mutation theory of de 

 Vries may be indicated as follows : — 



a. New species arise abruptly regardless of environ- 

 ment without transitional forms, and at present they 

 are not known to arise in any other way. 



h. New forms arise as unusual deviations from the 

 parent form, which itself remains unchanged, al- 

 though it may repeatedly give rise to similar devia- 

 tions. 



c. New mutations are, from the first, constant, that 

 is, they produce their like. They do not become 

 gradually established as t^e result of natural selection. 



d. Among mutations there may occur forms 

 characterized by the addition of something new, — 

 progressive elementary species, — as well as forms 

 lacking something present in the parental type, — 

 regressive varieties. 



e. The same mutation may arise simultaneously in 

 many individuals instead of as a single '' sport." 



/. Mutations do not vary around an arithmetical 

 mean with respect to the parent form, as is the case 



