VIRUSES, RICKETTSIAE AND BACTERIA 21 



There are two main views concerning the nature of viruses. One 

 suggests that they are in fact the simplest and most primitive 

 forms of living material and that originally they utilised the pro- 

 teins found in the complex primaeval " soup." As they gave rise 

 to more complex living things which altered and destroyed the 

 primaeval soup, so they became obligate parasites in other living 

 systems that evolved along different lines. On the other hand there 

 is the view that the viruses arose from more complex systems and 

 that in effect they are more like genes that have taken on a free- 

 lance life. Both of these views are discussed by Luria (1953). 



There are other opinions concerning the nature of viruses. Thus 

 Hadzi (1953), for example, has suggested that viruses are the 

 spores of parasitic Protozoa. It is possible that all these opinions 

 are correct and that the viruses are a complex group of substances 

 at present classified by their properties and that these properties 

 depend on the level of organisation that has been achieved. The 

 viruses are thus most likely a grade of organisation that has been 

 reached from many different directions. 



In this context and throughout the book, a grade may be re- 

 garded as a group of individuals that are united by certain common 

 properties but are not derived from a common close ancestor. 

 The grade indicates the level of organisation rather than a close 

 phylogenetic relationship. 



The rickettsiae 



The rickettsiae cause such diseases as typhus, murine fever and 

 spotted fever. They have properties between those of bacteria 

 and viruses; they approach the bacteria in structural complexity 

 and size, and they resemble viruses in that they are unable to 

 reproduce outside living cells (though this is not a stringent 

 criterion; it merely indicates lack of experimental success so 

 far). 



The rickettsiae are more complex than viruses in that they are 

 able to carry out certain of the metabolic processes of the higher 

 cells. Thus they are capable of oxidising glutamate, pyruvate, 

 succinate, fumarate and oxalo- acetate. These substances are also 

 oxidised by the mitochondria of the normal cell, and the sug- 

 gestion has been made that the rickettsiae are in fact free mito- 

 chondria. Thus both the mitochondria and rickettsiae lose their 



