128 C. A. KNIGHT 



generally recognized that viruses were a distinct group of disease agents. 

 Pasteur and his associates (1884) reported that intracerebral passage of the 

 rabies agent through a series of rabbits resulted in a substantial decrease in 

 the virulence of the rabies virus for its common host, the dog. Such a change 

 persisted, that is, it was heritable, and the attenuated virus was designated 

 "virus fixe" or "fixed virus." Since that time, mounting evidence has shown 

 that variation is a very common characteristic of animal viruses. Indeed, this 

 biological variabihty is both a blessing and a bane, for some of the most 

 successful vaccines used to combat viral diseases of man and other animals 

 have been made from attenuated viruses; while, on the other hand, the inevit- 

 able rise of new variants continually threatens the efficacy of these same 

 vaccines. 



Many years after Pasteur's experiments with rabies virus, Carsner and 

 Stahl (1924, 1925) suggested that a similar attenuation of the virus of the 

 curly-top disease of sugar beets was obtained by passage through nettle- 

 leafed goosefoot, Chenopodium murale. Shortly afterwards, McKinney (1926) 

 described the appearance of yellow spots on green mosaic-diseased tobacco 

 plants. Transfers could be made from such spots to healthy plants where 

 yellow mosaics were obtained rather than the green mosaics commonly 

 associated with tobacco mosaic disease. These yeUow mosaics were later 

 suggested by McKinney (1929) to be variants of the mosaic virus and such a 

 conclusion has been abundantly supported in the ensuing years. 



These key observations on sugar beet cuxly top and tobacco mosaic viruses 

 seem to have catalyzed a multitude of discoveries of plant virus strains, 

 especially during the period 1930-1940. The investigations of Kunkel (1940) 

 and Jensen (1933) revealed an astonishing number of mutants of tobacco 

 mosaic virus (TMV), and similar findings, although usually on a smaller 

 scale, were made over and over again with a wide variety of plant viruses 

 (Kunkel, 1947), It seems safe to conclude now that few, if any, plant viruses 

 do not mutate. In fact, the recognition that aU viruses contain genetic 

 material provides almost by definition that viruses should show mutations. 



B. Terminology 



At this point it seems appropriate to discuss briefly terminology. Some 

 workers speak of variation and others of mutation of viruses. Quite often the 

 terms are used interchangeably. If mutation is considered to be a sudden 

 random change in the hereditary material of an organism as a result of which 

 the offspring bear a heritable characteristic different from the parents, it is 

 clear that viruses mutate. There seems little basis at present for distinguish- 

 ing between mutation and variation. The product of a heritable change in 

 plant viruses is referred to as a mutant or variant, or very commonly as a 



