212 GENETICS AND EUGENICS 



plants, real genetic changes are involved whenever selection 

 on the basis of vegetatively produced individuals or struc- 

 tures is found to change the racial character. Such a rela- 

 tion has been observed to hold in all cases thus far carefully 

 studied. 



In regard to variegated seed-coat in maize, Emerson and 

 Hayes are agreed that the chief genetic changes occur in one 

 and the same gene, which results in producing a series of 

 multiple allelomorphs. Hayes recognizes four allelomorphs 

 in the same series, Emerson "at least nine or ten." The num- 

 ber is probably limited only by the ability of the observer 

 to discriminate them. Besides variation in a single gene, 

 Hayes assmnes additional "slight germinal variations," 

 probably to be understood as changes in other genetic loci, 

 possibly located in other chromosomes and functioning as 

 "modifying factors." Emerson finds that some states of 

 the chief gene for variegated seed-coat in maize are appar- 

 ently more stable than others, since some members of the 

 multiple allelomorph series are observed to mutate less fre- 

 quently than others. Thus, "self-colored and colorless races 

 are," he says, "as constant probably as most Mendelian 

 characters," but the truly variegated or intermediate types 

 mutate much more frequently, from one type of variegation 

 into another, or even into the more stable self-colored and 

 colorless types. 



Parthenogenesis in animals, like vegetative reproduction 

 in plants, when as commonly it occurs without the forma- 

 tion of gametes, affords an opportunity to observe how com- 

 mon genetic changes are. For in such cases no reduction of 

 the chromosomes occurs, there is no segregation of duplicate 

 genes, and there is no opportunity for the production of new 

 character combinations as a result of union of gametes in 

 fertilization. Genetic changes can in such cases occur only 

 under conditions comparable with those of bud-variation in 

 plants. Banta has observed for long periods, extending into 

 hundreds of generations, the successive parthenogenetic gen- 

 erations of small Crustacea known as water fleas (Simoce- 



