162 III. THE EVOLUTION OF VIRUSES 



and by the symptoms they cause on tobacco and tomato plants. All 

 of them at normal temperatures produce only necrotic lesions on the 

 latter, but some strains produce smaller lesions than others. All 

 cause local lesions on tobacco plants, but some cause necrotic lesions 

 and others chlorotic. Also, some give systemic infection and others 

 do not. According to Kunkel (76), over 400 different strains of tobacco 

 mosaic virus have already been recognized. And yet, still new strains 

 are continuously being found to increase the number. 



Of animal viruses the individuality or variability seems to be 

 especially distinct in influenza virus. Sigel (77) stated that strains of 

 influenza type A virus which originated in the same institutional out- 

 break from specimens collected on the same day, treated in the same 

 manner, and isolated by individual procedures, exhibited certain bio- 

 logical differences ; namely, dissimilarities in the capacity to aggluti- 

 nate chicken and guinea-pig red cells, speed of elution from the red 

 cells, and resistance to ultraviolet light. Even significant differences 

 in antigenic composition were found. 



As already mentioned, from various sources of animals have been 

 frequently isolated agents that cause pathological symptoms, when 

 transferred to proper other animals, like those of influenza or of en- 

 cephalitis, suggesting that normal or pathological protoplasm particles 

 of some animals tend to give rise to the pathological symptoms in 

 other animals. Such agents may vary in their properties with animals 

 or organs from which they have been generated, and even with the 

 same organ of the same animal, considerable differences should be ex- 

 pected as with tobacco mosaic viruses or phages in chicken feces. If 

 an agent is forthcoming having a striking affinity and a high viru- 

 lence to a man, then a new epidemic of influenza or of encephalitis 

 may follow. As is generally acknowledged, from every epidemic of in- 

 fluenza can be isolated a strain which differs in some or other aspects 

 from any strains ever isolated from other epidemics. It is also known 

 that new strains of neurotropic viruses causing symptoms like those 

 of encephalitis or of poliomyelitis are continuously being isolated from 

 various sources. 



There is a rare disease called postvaccinal encephalitis, which may 

 arise a short time after vaccination against small pox or rabies. It 

 should be expected that agents above considered are sometimes con- 

 tained in the vaccines, the emulsions of animal cells. It may be pos- 

 sible that by such agents postvaccinal encephalitis is caused in indi- 

 viduals with high susceptibility to the agent. 



Following a variety of infectious diseases such as measles similar 

 encephalitis is known to occur in which a virus causing the cencepha- 

 litis generated in certain cells of the patient by the stimulus of the 



