222 COOK 



in the first or conjugate generation, may be expected to be con 

 stant or " true to seed " when they have once gained expression. 

 In the language of special students of Mendelism, such a muta- 

 tion is already a " pure recessive." To secure equally uniform 

 expression of the dominant character it may be necessary to 

 extract the " pure dominants," that is, to select the progeny of 

 parents which produce offspring of the dominant type only. 



A dominant mutation will usually make its first appearance 

 as a single individual. A new variation occurring at mitapsis 

 would be represented by only one female gamete, or by four 

 male gametes. A recessive variation may secure simultaneous 

 expression in numerous individuals of the same generation, or 

 may continue to gain apparently independent expression in 

 successive generations. The method of narrow breeding or 

 self-fertilization used by De Vries and other experimenters to 

 insure the purity of their stocks, is well calculated to bring such 

 abeyant or recessive variations into expression. 



These considerations may have practical significance in ex- 

 periments for inducing new mutative variations by hybridiza- 

 tion. The first or conjugate generation does not represent the 

 final result of the cross, but only the conjugate relations of the 

 gametes. The variations may be expected to take place in the 

 mitapsis which precedes the formation of the sex-cells for the 

 next generation, but only the dominant variations can gain ex- 

 pression in the second or perjugate generation of the cross. 



For a recessive variation to be expressed it would be neces- 

 sary for two of the recessive gametes to unite in conjugation, 

 and this could not occur in the second generation, in an}- sexu- 

 ally differentiated species. To insure the expression of the 

 recessive variations which the cross ma}' have induced, it would 

 be necessary to self- fertilize the perjugate generation. The 

 usual custom of breeders of plants propagated by buds or cut- 

 tings is to make their selections and close the hybridizing ex- 

 periment wdth the second generation. The expression of the 

 recessive variations is thus entirely prevented, for no third 

 generation is produced. With seed-propagated species the 

 diversity caused by the recurrence of reversions continue for 

 numerous generations, until the desired characters have been 

 •' fixed " by a course of persistent selection. 



