Control of Heredity: Eugenics 263' 



which he has described are due to segregation and recombi- 

 nation of old factors rather than to the appearance of new 

 ones. Indeed Davis has made up an Oenothera by hybridiza- 

 tion that is very similar to O. lamarckiana. There are many 

 things which seem to indicate that this species and probably other 

 species of Oenothera are not genetically pure and it is probable 

 that some of deVries' results may be due to this fact. But it is 

 certain that mutations do take place in species where there is no 

 evidence of genetic impurity, as for example in Drosophila me- 

 lanogaster, and it is an extraordinary circumstance that some at 

 least of the mutations of Oenothera upon which deVries founded 

 his great theory are probably not mutations at all but are, as 

 Muller has said, "merely the emergence into a state of homozy- 

 gosis, through crossing over, of recessive factors constantly pres- 

 ent in the heterozygous stock." DeVries himself suggested this 

 explanation for his double reciprocal crosses and as Muller says, 

 "it probably lies at the root of nearly all the unusual genetic phe- 

 nomena of this genus." That there are lethel factors also in 

 Oenothera which produce their effects upon the gametes rather 

 than the zygotes is indicated by the partial or complete failure 

 to form fertile pollen in certain forms of this genus. 



It is probable that many natural or Linnean species, other than 

 O. lamarckiana, are not pure and homozygous; within every such 

 species there are usually found many "elementary species" and 

 by intercrossing of these a mixture of many lines or strains re- 

 sults from which new forms may occasionally arise by segre- 

 gation. Lotsy maintains that all mutations arise in this way. 

 But such an explanation does not account for the existence of 

 the original "elementary species" and if they be referred to still 

 earlier crossings it is evident that we only put off the explanation 

 to a more remote period. On the other hand the exact and ex- 

 haustive work of Morgan and his associates on Drosophila has 

 proved that the mutations in this species are not due to such 

 segregation and it plainly indicates that they are caused by sud- 

 den transformations in the Mendelian factors themselves, com- 

 parable to changes in chemical composition. 



