THE NATURE OF VIRUSES 967 



1938, Laidlaw 1938, Gortner 1938) is that the viruses represent decadent forms 

 of organisms that have become progressively degraded through long persistent 

 parasitism. At the up23er end of the scale are representatives, such as the psitta- 

 cosis and the vaccinia viruses, that contain protein, fat, and carbohydrate, and 

 that differ from bacteria chiefly in their loss of power to synthesize some factor 

 or factors essential for their growth and multiplication : at the lower end of the 

 scale are representatives, such as the tobacco mosaic virus, that consist of pure 

 nucleoprotein, and that are entirely dependent on living cells for all their ferment 

 and autosynthetic activities — inert chemical complexes which become living only 

 when bathed in functioning protoplasm. This view has something to be said for it ; 

 it serves to reconcile the apparent break between the living and the non-living. 

 It is, however, not without difficulties of its own. Since the tobacco mosaic virus 

 can grow in a number of plants, belonging even to different families, in which the 

 composition of the cell protoplasm is known to be different, it follows that its 

 huge nucleoprotein molecule must be built up from relatively simple chemical 

 substances. This presupposes a degree of synthetic organizing ability hitherto 

 associated with living cells alone. Acceptance of this view opens up once again 

 the whole question of spontaneous generation. If a protein molecule can, in con- 

 tact with living matter, reproduce itself and undergo variations each of which 

 is attended by specific biological changes and each of which is genetically trans- 

 missible, it is difficult to avoid the conclusion that new bodies, presenting the 

 characters of living matter, must be constantly appearing in Nature. How also 

 are we to regard structure in relation to size ? The foot-and-mouth virus, for 

 example, which is about 12 m// in diameter, is so similar in almost every respect 

 to the ■'drus of vesicular stomatitis, which has a particle size of 85 m^, that it is 

 difficult to avoid the conclusion that they are structurally alike. Yet the foot- 

 and-mouth virus is smaller — at least in one diameter — than the tobacco mosaic 

 virus, the length of which is estimated to be about 30 m^. Must we assume that 

 the foot-and-mouth virus, which incidentally has room for only about ten protein 

 molecules, is organized like one of the larger animal viruses, in spite of its being 

 smaller than a plant virus which is known to consist of a single macromolecule ? 



One thing is clear. Before we can reach any conclusion on the nature of 

 filtrable viruses, we shall have to re-define our terms. At the larger protein level 

 the words " living " and " non-living " have lost their conventional meaning (see 

 Pirie 1937). It is difficult, even in Science, to avoid the common solecism of 

 attempting to force new facts into a conception that has no reality as such, but has 

 been formed merely as an abstraction from other previously known facts ; and it 

 is time for us to realize that our concept of " life " is too crude to be used in relation 

 to the infinitely small. Whatever the viruses are — micro-organisms reproducing 

 themselves by binary fission, huge nucleoprotein molecules multiplying by auto- 

 catalysis, or something else still — there is no doubt that they present us at the 

 moment with one of the most fascinating and fundamental problems of the 

 biochemical world. 



We have said nothing in this chapter about the filtrable tumours. Here 

 again we are on very difficult ground. There is evidence suggesting that virus 

 particles are essential to their reproduction, and that tumours both of avian and 

 mammalian origin are caused, to some extent at least, by infecting particles 

 having many of the characteristics of the known filtrable viruses (see Gye 1925, 

 Andrewes 1934, Ledingham and Gye 1935, and Chapter 89). 



