VI. FACTORS INTERFERING WITH GENES 273 



nation by means of the sexual reproduction of the host appears to be 

 easily attainable for the viruses and accordingly to be most conven- 

 tional for them to continue their existence. 



As for the viruses affecting man, most of them can infect solely 

 some particular individuals, though some of them, such as those of 

 measles and mumps, are capable of affecting almost all the individuals. 

 These latter viruses seems to be able to engrave their pattern into 

 the germ cells of almost all the human beings, whereas some other 

 viruses appear to do so only in particular persons who have the 

 predisposition liable to be afflicted by them. The pattern of the 

 viruses will be transmitted to the children of such peculiar persons 

 affected by the viruses, and the pattern will be developed into its full 

 form when the children grow up to certain ages, leading them to the 

 disease or rendering them virus-carriers, who will subsequently scatter 

 about the viruses. Some of the viruses thus scattered about may 

 affect individuals having the peculiar predisposition, and through such 

 individuals they can be rejuvenated and transmitted to offspring. The 

 viruses must be extinguished if there are no such predisposed per- 

 sons. 



Even in the case of measles, about 5 per cent of man are not 

 infected with the virus for life. Such individuals must be completely 

 freed from the pattern of the virus, since the virus cannot enter their 

 germ cells. Measles would be expelled from this globe if all the 

 human beings had such a peculiar character. 



In addition, climatic or topographical conditions may play a con- 

 siderable role in the development of virus diseases. Persons living in 

 a certain region may have a peculiar disposition according to the 

 climatic factors in the region and may be liable to be afflicted by 

 some virus which cannot, or scarcely, infect the man living in other 

 regions. In such a case the virus disease may be called an endemic 

 disease, the pattern of the virus in question being engraved only in 

 the germ cells of man living in such a region. This may also hold 

 for animals other than man, and so it may occur that certain insects 

 or other animals in a certain region always prove to be carrying a 

 virus peculiar to the region. 



The development, in a child, of the full pattern of the virus which 

 is transmitted from parent appears to be effected mainly by the age 

 of the child. However, many other factors are apparently involved 

 in the development. It is a remarkable fact, as already discussed, 

 that leaf hoppers carrying aster yellow virus lose the virus if exposed 

 to a temperature of 32^C. (81). This fact can be interpreted as indi- 

 cating that the structure of the virus cannot develop at Z2~C. 



The incidence of measles changes regularly with the change of 



