Control of Heredity: Eugenics, 279 



only when this linkage is broken that they have a chance to 

 develop. 



However, it is certain that mutations do take place in species 

 where there is no evidence of genetic impurity, as for example in 

 Drosophila melanoga-ster, and it is an extraordinary circumstance 

 that many of the mutations of Oenothera upon which deVries 

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

 as Muller (1917) has said, "merely the emergence into a state of 

 homozygosis, through crossing over, of recessive factors constantly 

 present in the heterozygous stock." DeVries himself had pre- 

 viously suggested this explanation for his double reciprocal crosses 

 and as Muller says, "it probably lies at the root of nearly all the 

 unusual genetic phenomena of this genus." That there are lethal 

 factors also in Oenothera which produce their effect upon the 

 gametes rather than upon 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. After all, the fundamental problem 

 here concerns the origin, rather than the segregation, of domi- 

 nants, recessives, lethals, etc. The exact and exhaustive work of 

 Morgan and his associates on Drosophila has proved that the mu- 

 tations in this species are not due to Mendelian segregation and it 

 plainly indicates that they are caused by sudden transformations 

 in the Mendelian factors themselves, comparable to changes in 

 chemical composition. 



