156 GENETICS 



zation is not sought for among phenotypes, but 

 among genotypes. It is not the somatoplasm, but 

 the germplasm that is selected. 



/. The^M^wd^E^jdm^ 

 is a method of greatly increasing the number of 

 variants- by promiscuous hybridization^ and then of 

 .eliminating all except those of a desired phenotypic 

 combination. Indirectly it depends upon the princi- 

 ple of the segregation of unit characters which makes 

 possible rearrangements of these characters according 

 to the laws of chance. The characters themselves 

 remain unchanged, since nothing new is produced 

 by hybridization except new arrangements of existing 

 characters. 



The spectacular success of Luther Burbank in 

 "creating" new plant forms is due largely to his very 

 extensive hybridizations, his skill in detecting among 

 the varying progeny the winning phenotype and his 

 ruthless elimination of the great majority of varia- 

 tions that do not quite fill his requirement. 



The successful combinations must be propagated 

 in most instances asexually by grafting, cuttings, 

 bulbs, etc., rather than sexually through the medium 

 of seeds, because new genotypes which will breed true 

 are- not necessarily isolated by this procedure. The 

 . Inconsequence is that Burbank's method cannot be 

 \llutilized in animal breeding to any great extent where 

 Wthe maintenance of a desirable strain by asexual prop- 

 agation is out of the question. 



It will be seen that this method, like the first two, is 





