206 III. THE EVOLUTION OF VIRUSES 



a similar way viruses have evolved from higher organisms as a 

 result of degeneration due to the parasitism. It seems, however, that 

 this idea might be based on a grave misunderstanding as regards 

 parasitism and the basic principles of life phenomena as well. 



It is true that parasites seem to be often very highly modified in 

 their structure to meet the demands of their particular enviroment. 

 Fixed parasites generally have neither sense organs nor well-developed 

 locomotive functions, and intestinal parasites do not need highly 

 organized digestive tracts. However, parasites must be specialized, 

 often to a very high degree, to adhere to or to make their way in 

 their particular host, or the particular part of the host in which 

 they find suitable conditions for their existence. Still more remar- 

 kable are the specialization of parasites in their reproduction and 

 life history to insure, as far as possible, a safe transfer to new hosts 

 for succeeding generations. .Every structure, every function, every 

 instinct of many of these parasites appears to be modified, to a certain 

 extent, for the sole purpose of reproduction. 



The apparent degenerancy associated with parasitism is, there- 

 fore, nothing but the highly developed specialization wonderfully 

 adapted to the peculiar, extremely limited, environment. As above 

 cited, viruses are distinct from typical organisms in their lack of 

 self- or race-preservation instinct, whereas in ordinary parasites this 

 instinct is found to be most manifest as just stated. 



4. Inclusion Bodies and Metamorphosis 



Many virus infections are characterized by the presence of peculiar 

 bodies, usually referred to as inclusion bodies, in the nucleus or cyto- 

 plasm of parasitized cells. The bodies are rounded, oval, or irregular 

 in shape. It has proved possible to photograph with the electron mic- 

 roscope inclusion bodies such as those of mouse ectromelia and human 

 variola, and they are seen to consist of aggregations of virus particles 

 clumped together. Thus it is difficult to regard inclusion bodies as 

 other than intracellular aggregations of virus particles. 



It has already been stated that plant-protoplasm particles are 

 liable to aggregate into larger bodies sometimes resembling protozoa. 

 This may be regarded as the production of inclusion bodies in vitro. 

 The inclusion bodies may be produced in the host cells when virus 

 structure is enveloped by the normal structure of the protoplasm ; 

 since both structures can not melt into each other because of their 

 different structures, the virus structure may have to exist in forming 

 a mass distinctly separated from the host protoplasm. 



