CHAPTER XIX 
HYBRIDIZATION 
The usual purpose of the plant breeder who resorts to hybridiza- 
tion is to secure new and better combinations of characters among the 
progenies resulting from his crosses. Improvement of a given species 
may consist merely in the elimination of undesirable characters or of the 
production of entirely new combinations of characters already existing 
within the species. In this work the application of the Mendelian 
principle of segregation and recombination is of the greatest prac- 
tical value. By concentrating his attention on only a few important 
characters at a time the breeder can sometimes secure the desired 
combinations in Ff, But at the same time one who is informed in 
regard to modern genetical principles will be prepared for possible 
disappointment in meeting an early realization of his aim. Moreover, 
he will understand how to select in /’, and later generations for further 
testing. He will realize that a specific character difference in his parental 
forms may be conditioned by more than one factor difference; also that 
some specific factors display considerable variability in expression; and 
that linkage, crossing-over, multiple factors and multiple allelomorphs 
may play a réle in conditioning or preventing the particular character 
combination for which he is striving. Furthermore, the ideal sometimes 
demanded of the breeder involves character complexes which include all 
the functions of the plant. As has been shown already the comparative 
difficulty between different cases of this sort depends directly upon the 
number of chromosomes possessed by the species in question. Finally, 
demands are sometimes made for the “creation” of characters which are 
unknown in available phenotypes and for which there is no genotypic 
representation within the species. In such cases recourse may be had 
perhaps to species hybridization. But those who are familiar with the 
results of species hybridization will be prepared for complete disappoint- 
ment from the first. It is not the motive of these remarks to discourage 
intending hybridizers, but merely to warn against the anticipation of 
success in all cases simply because of the generality of Mendelian princi- 
ples. Hybridization, even of varieties, in order to be generally successful 
must be intelligently performed and in the long run the experimentalist 
who is the most thoroughly informed concerning his plants will stand the 
best chance of securing the improved forms he desires. Each species 
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