408 GENETICS IN RELATION TO AGRICULTURE 
in each we find the same diversity among natural species as regards 
disease resistance. It is unnecessary to multiply instances further. In 
all likelihood the resistance of the Chinese chestnut to Endothia parasitica 
and of the Chinese Sand Pear to the fire-blight bacillus is due to some 
specific quality of the protoplasm probably something in the nature of 
an antitoxin. That this quality is heritable will be seen from the results 
of hybridization experiments. 
Breeding Disease-resistant Varieties by Hybridization.—Allusion 
was made in Chapter XX to the fact that first generation maize hybrids 
are often more drouth resistant than either parent. Presumably this 
is merely one manifestation of heterosis. Hybridization is a very im- 
portant means, however, for the production of improved varieties which 
are better adapted to specific adverse elements of the environment. 
Witness the important results already secured in the production of cold- 
resistant varieties of fruits, grains and forage plants, by Hansen, Patten 
and Saunders and at the U. 8. Agricultural Experiment Stations in 
Alaska. 
At one stage in the anti-phylloxera campaign in France and California 
viticulturists held definitely to the ideal of securing through hybridi- 
zation ‘‘a vine that, while resisting the phylloxera, the two mildews, 
the black rot, ete. (all of which diseases are natives, and which the 
American vines resist more or less well), will give without grafting a grape 
that has size and the quantity and quality of the Vitis vinifera.” With 
this object in mind many crosses were made but they have produced no 
hybrids between vinifera and American species that can be substituted 
for the choice vinifera varieties. It, therefore, became necessary to util- 
ize resistant species and hybrids as stocks on which to graft the pro- 
ducing varieties. However, it is still possible that, by growing large 
numbers of F, and I’; seedlings from some of the most promising F 
hybrids, the dream of the viticulturist might be realized. It seems that 
no grape breeders have carried out extensive tests of hybrids beyond the 
first generation from the cross. This is not strange inasmuch as grape 
breeding for phylloxera resistance was at its height during the latter part 
of the 19th century and before the importance of testing for several 
generations after a cross was generally appreciated. That phylloxera re- 
sistance and susceptibility are conditioned by specific genotypic elements 
is evidenced by the results of Rasmuson who tested J’, seedlings from 
several crosses between certain American species and between American 
species and V. vinifera, as well as crosses between different varieties of 
vinifera. The latter, he reports, yielded only susceptible offspring while 
the crosses between different American species gave both resistant 
and susceptible offspring, the latter being in the minority. Resistance 
appeared to be dominant and susceptibility recessive in the progeny of 
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