168 III. THE EVOLUTION OF VIRUSES 



while the other old, generated much longer before. The particle size 

 of virus may present some indication ; the virus of foot-and-mouth 

 disease and of poliomyelitis, and some kinds of plant viruses have been 

 reported to be the smallest in size. Such viruses may be regarded as 

 possessing comparatively primitive, undifferentiated structures, since 

 "small size" may be interpreted as indicating that the virus or the 

 assimilase can maintain its activity even when decomposed to this 

 small size. Indeed, such small-sized viruses seem liable to develop de 

 novo. Coxackie virus which bears a strinking resemblance to polio- 

 myelitis virus has been reported to have the size of 10 to 15 m/i (86) 

 indicating that this virus is also one of the smallest viruses. On the 

 other hand, the so-called lymphogranuloma-psittacosis group viruses 

 are generally accepted to be most differentiated ones, and they are be- 

 lieved to be as large as 250 to 500 m^. Nevertheless, there appears 

 not always such correlation between differentiation degree and the 

 particle sizes. For instance, influenza virus, spontaneous generation 

 of which appears most possible, is generally acknowledged to be in 

 size about 100 m/i, whereas with yellow fever virus which appears to 

 be a fixed virus such small figure as 22 m// has been reported (87). 

 As already stated, since the size of a virus may be determined by the 

 property of the protoplasm which the virus has affected, and also by 

 the smallest protoplasm particles, in which the virus activity can re- 

 main, no great significance may be expected in the size. 



However, fixation of the character of a virus as an independent 

 entity seems to be accompanied by the establishment of character to 

 form uniform particles, and moreover the complexity of the activity 

 necessarily associated with the development of differentiation in virus 

 may require large sizes because small particles may be impossible to 

 contain the complex structure. Consequently, large uniform sizes may 

 commonly be associated with old and differentiated viruses. In the 

 case of Rickettsiae, as referred to in the next chapter, this seems es- 

 pecially the case. 



