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, 



