176 III. THE EVOLUTION OF VIRUSES 



mitted to other organisms with vigorous multiplication, only thereby 

 variations in a desirable direction become possible. Remaining in the 

 same host must result in its extinction together with the host as in 

 the case of tumours. 



On the other hand, In the case of common cold, for instance, the 

 newly formed structure carried by particles may be able to spread 

 easily through the coughing. The fact that common cold is always 

 accompanied by coughing may enable the virus to exist for consider- 

 ably prolonged periods and thus the virus may be able to become more 

 and more virulent until it may come to appear even to be a kind of 

 influenza virus. 



Like the virus of common cold, a large number of viruses, such 

 as those of measles, m.umps, poliomyelitis, and influenza, may be 

 spread, though not so readily, by means of droplets of secretion from 

 the upper respiratory tract. In the case of venereal diseases, such as 

 lymphogranuloma inguinale, spread of viruses by direct contact can 

 be afforded. In plant virus diseases, infections by direct contact is 

 also conceivable. 



Many other ways may be possible for viruses to spread to other 

 host individuals, but in this connection a strong reason for which the 

 continued existence of viruses may become most difficult can be con- 

 sidered. The reason is that, for the continued existence, viruses must 

 be transmitted frequently to other organisms quite different from the 

 customary host. 



As mentioned already, when a virus continues for a long time to 

 multiply by successively affecting a series of the same kind of host, 

 the virulence in the long run has to decrease until the virus itself is 

 lost, because the virus will be gradaully assimilized by the host pro- 

 toplasm to become quite identical to the latter. In order to avoid 

 such a fate, viruses have to change occasionally their host. Insects 

 appear to be universally utilized by viruses for this purpose as will 

 be detailed in the next chapter. 



