DISEASE AND RELATED PHENOMENA IN ANIMAL BREEDING 529 



which, although as thin as that of domestic cattle, is more difficult to 

 pierce; and finally the short coat, which does not provide shelter for 

 the ticks. The bison also appears to be immune to the attacks of the 

 same pest. 



Breeding for Immunity. The matter of breeding for immunity to 

 disease is one which may have tremendous practical importance in animal 

 breeding. Success in such breeding depends upon the existence of strains 

 or races of immune animals, and upon the transmission of these characters 

 to the offspring. Thus in Tyzzer's experiments with mice it was found 

 that the Ehrlich tumor, which made only a very insignificant growth in 

 Japanese mice, also failed to develop in the FI hybrids between common 

 and Japanese mice. In the same fashion FI mice of the cross common X 

 Japanese proved to be resistant to inoculation with the Jensen tumor, 

 in this respect resembling the Japanese parent. On the other hand FI 

 mice of the cross common X Japanese were very susceptible to inocula- 

 tion with the Japanese tumor, even more so than the Japanese mice 

 themselves. Here the immunity of common mice to inoculation does 

 not appear to have been carried over to the hybrids. The F 2 of this 

 cross, however, behaved very peculiarly. Of fifty-four F 2 individuals 

 not one proved susceptible to the tumor, and sixteen F 3 individuals 

 gave like negative results. The transmission of immunity to disease is 

 established by these experiments, but the exact factor relations cannot 

 be stated. 



In domestic animals the possibility of breeding for disease resistance 

 has long been held in mind, and in some cases steps have actually been 

 taken in that direction. In the Southern States, particularly in Texas, 

 Texas fever has annually levied its millions of dollars' tribute upon the 

 cattle industry. The trouble has apparently been intensified since the 

 attempt to grade up range cattle by the use of pure-bred Hereford and 

 Shorthorn bulls, for these improved types of Northern cattle are more 

 susceptible to tick infestation and splenetic fever than their hardier, 

 but otherwise less desirable, range relatives. Apparently there are some 

 grounds for the belief that range cattle have received infusions of zebu 

 blood from early Spanish importations into Mexico and from zebu cattle 

 brought to South Carolina in 1849. Accordingly it is not impossible 

 that the comparative freedom from tick and insect infestation which is 

 characteristic of unimproved range cattle, has come originally from the 

 zebu. 



As in our own domestic cattle so in the zebu there are many different 

 breeds, and they present differences no less striking than those of the 

 Northern cattle. They have been bred for centuries under the tropical 

 conditions of India and Africa. They are disease resistant; they are 

 able to withstand tick and insect pests to which the Northern breeds 



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