528 GENETICS IN RELATION TO AGRICULTURE 
a male and female Gujarat zebu among seventy brown Alpine cows were 
the only individuals spared by the foot-and-mouth disease. 
There is another type of immunity which is characteristic of certain 
individuals within a race. It is a matter of common observation that 
individuals occasionally appear which are completely immune to a given 
disease. It is difficult to state precisely upon what this immunity 
depends, but it is none the less definite, and it is apparently often heritable. 
When heritable it may under appropriate conditions become a racial 
character. It has been thought that upon this depends the comparative 
immunity which certain races bear against given diseases. The negro of 
the West Indies is comparatively immune to the ravages of yellow fever, 
presumably because for centuries the more susceptible individuals have 
succumbed to the disease, so that the race has been propagated for the 
most part by less susceptible individuals or those which survived the 
disease. The white man on the other hand is more susceptible to yellow 
fever because no process of selection has weeded out susceptible strains. 
Measles, also, is considered a very mild disease among Caucasian peoples, 
but among the North American Indians it is very severe, spreading 
through tribes like a veritable plague and proving fatal in many cases. 
There is another type of immunity which is acquired by the individual 
during life. Persons who have had smallpox have had conferred upon 
them an immunity which lasts for several years, and the same is true 
of other diseases in man and in other animals. This type of immunity 
may be induced artificially in the individual by appropriate treatment 
such as is done in vaccination, the administration of anitoxins and other 
forms of immunization. In animals the practice is seen in the distribu- 
tion by experiment stations of blackleg vaccine for calves and hog cholera 
serum for swine plague. 
A type of indirect immunity is that of resistance to attack by agents 
carrying a particular disease. Thus yellow fever is carried by a certain 
kind of mosquito, Aedes calopus, and malaria by certain species of the 
genus Anopheles. It would be possible, therefore, for individuals to 
enjoy freedom from the attacks of either of these two diseases if they hap- 
pened to be resistant or repellent to the attacks of the particular mos- 
quitos which carry the disease. Sometimes active immunity is associated 
with such resistance to attack. This matter does not look so strange 
when it is recalled that often a very specific relation exists between 
parasite and host among animals, and that very often diseases are trans- 
mitted by insects and other animal pests. In animals immunity of this 
kind is exhibited by the zebu against the Texas fever tick. According 
to Mohler the immunity which the zebu enjoys to tick infestation depends 
upon three factors; the sebum secreted by the glands of the skin, which 
has a repellent odor repugnant to insect life; the toughness of the skin, 
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