VIRUSl.S. I'RO VI RUSES AND THE CONTLICT OF SYSTEMS 



Certain stocks of flies were found to die in the presence of moderate 

 doses of carbon dioxide. This unique susccptibihty was a mere 

 biological freak which, in nature, neither helped nor hindered its 

 possessors. The property was inherited largely in the female line. 

 But it could be transmitted to a proportion of the offspring by the 

 sperm of susceptible males fertilizing normal eggs. It was ambilinear 

 although not cquilincar. The susceptible offspring, however, could 

 be cured by subjecting them to a temperature of 33"^ C. for 24 

 hours either immediately on laying or during pupation, i.e. during 

 the periods of most rapid cell-division. Thus the COg-sensitive 

 plasmagene failed to keep pace with the rest of the body proteins 

 at this temperature, while at a normal temperature the reverse was 

 the case: it was suppressive. 



So far we see a close analogy with peas and Paramecium. But there 

 is something more. Supernumerary sperm will carry the plasmagene 

 over to the normal egg. And finally, transplanted ovaries, or even 

 injected lymph, of susceptible flics will infect normal ones and the 

 susceptibility will show in a part of their progeny. In other words 

 the determinant is a provirus as well as a plasmagene. It must have 

 arisen by mutation in the ancestors of the susceptible stock and its 

 properties of infection and diffusion indicate that it is of no more 

 consequence in differentiation than it is in heredity. 



Neutrality again enables us to find an overlapping of the proper- 

 ties of heredity and infection in the case of the piebald guinea pig, 

 a fancier's mutant. The black pigment is produced both in the hairs 

 and in the skin of the coloured areas. Pieces of coloured skin grafted 

 in white areas lead to pigmentation of the adjoining unpigmented 

 cells and these will, in tum, infect more white cells after a second 

 transplantation. Thus, as Billingham and Medawar have pointed 

 out, there are cytoplasmic determinants, expressing hereditary 

 characters and capable of indefinite self-propagation, which never- 

 theless are capable of difflision, invasion or infection. It seems likely 

 that in animals, at least, invasions of this kind, but of limited scope, 

 are concerned in processes of normal differentiation. 



The Origin of Viruses 



In both animals and plants there are a number of diseases which 

 have not arisen by infection although they can certainly be propa- 



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