ANIMAL VIRUSES: A COMPARATIVE SURVEY 7 



same particle size and are pathogenic for suckling mice would almost demand 

 that these should be included in the group. 



H. Other Groups 



Apart from these major groups of virus, there remain many other forms 

 which have not yet been found susceptible to classification. A few may be 

 mentioned as perhaps representative of important groups. 



1. The avian viruses, producing intranuclear inclusions, of which infectious 

 laryngotracheitis is the prototype. 



2. Measles and canine distemper viruses, which may perhaps be the two 

 conspicuous members of a much wider group. 



3. Vesicular stomatitis of cattle and swine with the two serological types, 

 Indiana and New Jersey. 



4. It has been suggested that there may be a common character to the 

 viruses of infectious and serum hepatitis, rubella, and infectious mono- 

 nucleosis. 



5. The virus of neurolymphomatosis of fowls is accepted by Beard and 

 others as the prototype of a wide range of viruses responsible for proliferative 

 lesions in domestic poultry. It might be one of the most illuminating dis- 

 coveries in virology to find what form the ancestral forms of these viruses 

 take in wild birds. 



This still leaves the agents of one important human disease, rabies, and 

 many diseases of domestic and wild animals unclassified. Research has 

 naturally been concentrated on viruses responsible for human disease and on 

 those producing economically serious disease in domestic stock. Among these 

 the availability of convenient laboratory hosts has been largely responsible 

 for the choice of a very small proportion for detailed analytic studies. 



III. The Comparative Picture 



The picture that seems to emerge of the animal viruses as a whole is, 

 broadly speaking, not very dissimilar to that of any wide assemblage of 

 higher living forms, mammals or molluscs, for example. There are clearly 

 demarcated groups with basic similarities which have obviously nourished 

 and evolved to fill the available ecological niches. With the development of 

 tissue culture methods, we have been freed of the necessity of demanding 

 pathogenicity before a virus could be recognized and there has been an 

 almost embarrassing flood of virtually nonpathogenic forms, closely related 

 in many instances to viruses of classical pathogenicity. The extraordinarily 

 large number of "wild monkey kidney viruses" that Hull and others (1956) 

 have described and the various forms that have been isolated from experi- 

 mental mice in the last twenty-five years make it obvious that there are vast 



