210 GENETICS 



individuals of two varieties showing hereditary differ- 

 ences been hybridized but successful crosses have been 

 artificially brought about between individuals belong- 

 ing to different species, to different genera and even 

 to different groups still more distantly related to each 

 other. 



It may be possible to point out at least two general 

 methods of utilizing hybridization. 



a. The Method of Burbank 



This 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 principle 

 of the segregation of unit characters which makes 

 possible rearrangements of these characters according 

 to the laws of chance. The characters themselves re- 

 main 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 variations 

 that do not quite fill his requirement. 



The successful combinations mustjDe 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 



