APPLICATIONS OF PRINCIPLES OF HEREDITY 451 



By selecting individuals in this third generation and grow- 

 ing from them, and by keeping records of their behavior, he 

 succeeded in establishing strains that transmitted the desired 

 combination of characters. This is in accordance with the 

 Law of Unit Characters (see p. 448). 



In this way it was possible to combine in one variety of 

 wheat the strong stem, the closed head, and the winter-resisting 

 qualities needed for successful wheat farming in this region. 

 By similar methods it has been possible to combine three or 

 more characters desired in a plant or an animal from as many 

 different varieties of ancestors. 



474. Breeding for immunity. The chief problem of those 

 who have to do with plants and animals is to get organisms 

 that combine desirable qualities and show none of the unde- 

 sirable qualities. Thus, there is the American brand of cattle, 

 raised for beef on the large prairie ranches ; this has good 

 beef qualities and is very easily handled in large herds. But, 

 unfortunately, most of our cattle are very susceptible to the 

 destructive Texas fever, which has caused the loss of millions 

 of dollars' worth of cattle. It had been observed that the 

 so-called Brahmin cattle of India were immune to the Texas 

 fever. On mating an immune animal 'of this breed with one 

 of the susceptible varieties, it is found that the immunity is 

 dominant. A number of years ago a herd of the Brahmin 

 cattle was imported into this country for the purpose of cross- 

 ing with our native cattle, in order to establish a variety that 

 would have the beef qualities of the American cattle and 

 would at the same time be immune to the Texas fever. This 

 undertaking seems to produce successful results. 1 



1 In the meantime it has been found out that the Texas fever is transmitted 

 by a little animal known as the tick, which sucks the blood from the diseased 

 cattle. By suitable quarantine it has been possible to restrict the Texas fever ; 

 and by applying to the bodies of the cattle something that will either kill the 

 ticks or prevent their biting the cattle, it may be possible to eradicate this costly 

 disease. But if we could replace our present herds of cattle with a type that is 

 quite immune, the added cost would no doubt be made up in a very short time. 



