174 SECTIONAL ADDRESSES. 
the surrounding medium by a boundary membrane of special properties, 
necessary for the endowment of matter with any form of life? Or is it 
possible to conceive of a material complex, retaining in endless propagation 
its physiological character, as revealed by the closely specific reaction to 
it of the cells which it infects, though it is not organised into units, but 
uniformly dispersed in a watery medium ? Among those who study the 
viruses primarily as pathogenic agents, these questions provide matter for 
debate, the warmth of which may even penetrate our discussion of to-day. 
I suggest that they are questions with which the physiologist may properly 
be concerned. To-day, we may do little more than display the nature 
of the biological problem ; but I am not without hope that its discussion, 
by those who study diseases in animals and plants, may enlist the interest 
of physiologists in its solution. 
We cannot afford to-day much time for the history of the subject ; 
but it is of interest to note that Edward Jenner was dealing, in small-pox 
and vaccinia, with what we now recognise as characteristic virus infections, 
long before there was any hint of the connection of visible bacteria with 
disease. Pasteur himself was dealing with another typical case of a 
virus infection in the case of rabies. The clear recognition, however, of 
the existence of agents of infection, imperceptible with the highest powers 
of ordinary microscopic vision, and passing through filters fine enough 
to retain all visible bacteria, begins with Ivanovski’s work in 1892 on the 
mosaic disease of the tobacco plant, brought to general notice and greatly 
developed by Beijerinck’s work on the same infection some seven years 
later ; and with Loffler and Frosch’s demonstration, in the same period, 
that the infection of foot-and-mouth disease is similarly due to something 
microscopically invisible, and passing easily through ordinary bacteria- 
proof filters. Since those pioneer observations the study of viruses has 
spread, until they are recognised as the causative agents of diseases in an 
imposing and still growing list, containing many of the more serious 
infections of man, animals, and plants. 
If we are to discuss the biological nature of the viruses, it is obvious 
that we should begin by attempting some kind of definition. What do 
we mean by a virus, and what are the tests by which we decide that a 
particular agent of infection shall be admitted to, or excluded from, the 
group ? But a few years ago, I think that we should have had no difficulty 
in accepting three cardinal properties as characterising a virus, namely, 
invisibility by ordinary microscopic methods, failure to be retained by a 
filter fine enough to prevent the passage of all visible bacteria, and failure 
to propagate itself except in the presence of, and perhaps in the interior 
of, the cells which it infects. It will be noted that all three are negative 
characters, and that two of them are probably quantitative rather than 
qualitative. Such a definition is not likely to effect a sharp or a stable 
demarcation. We shall see that its failure to do so is progressive. 
Nevertheless, it would still be difficult to refuse the name of virus to an 
agent which fulfils all three criteria; and we must, therefore, in con- 
sistency, apply it, on the one hand, to the filtrable agents transmitting 
certain tumours, and, on the other hand, to the agents of transmissible 
lysis affecting bacteria, and now widely known and studied as bacteriophages. 
—— Se 
