184 III. THE EVOLUTION OF VIRUSES 



virus may not only be unable to remain for a long period but may 

 be defeated and assimilized soon by the normal structure. 



It is noted that not all larvae from eggs laid by insects carry- 

 ing a plant virus become infective, suggesting those of a weak pre- 

 disposition only admit the virus structure to develop. This appears 

 to hold true also for viruses transmitted by seed. The proportions of 

 infected seeds are reported to be highest v^rith plants which are them- 

 selves the progeny of infected seeds (39), a fact which is to be expected 

 since the predisposition associated with a weak structure may be 

 heritable. 



Not only the virus of measles but many other viruses of man 

 may, more or less, be heritable. For instance, mumps, a virus disease 

 of parotid gland, may be one of the heritable virus diseases. In this 

 connection, it should be remembered that in the submaxillar gland of 

 normal guinea-pigs can usually be demonstrated a virus, which, how- 

 ever, may become detectable only when the animals grow up to a 

 certain age. 



Viruses such as those of measles and mumps may have extremely 

 powerful structure, so that they may be able to pass though germ 

 cells to progeny without being much concerned with the predisposition 

 of man, but in the case of many other viruses, persons with a weak 

 predisposition only may be affected and the structure may be trans- 

 mitted to the next generation only when the virus affects most sus- 

 ceptible persons. 



4. The Cause of the Fixation of Viruses 



Some viruses confer the animals they affected high degree of 

 specific immunity, occasionally lasting for the remainder of their lives. 

 The writer designates such viruses as fixed viruses, and holds the 

 opinion that the immunity to a virus is caused by the engraving of 

 the virus pattern into the protoplasm of the animal and that the 

 immunity continues as long as the pattern is present in the animal 

 (12). In other words, when some cells in an animal are endowed with 

 virus structure, the animal is said to be immune to the virus. Since 

 such cells are endowed with the structure of the virus itself, the 

 virus which may invade afterward from outside will be preferably 

 combined with the cells and assimilized without exerting any injuri- 

 ous effect. 



If this concept is correct, a virus will be detected in organisms as 

 long as the organisms are immune to the virus. In fact this is the 

 case with plants. It is confirmed that the persistence of active virus 



