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 Ff; hybrids between common 
and Japanese mice. In the same fashion F’, 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 F; 
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, of this 
cross, however, behaved very peculiarly. Of fifty-four F,. individuals 
not one proved susceptible to the tumor, and sixteen F; 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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