i i 
I.— PHYSIOLOGY. 179 
whether the series is real and continuous, or whether it is formed merely 
by the accidental association, through a certain similarity in effects, and 
- through common characteristics of a largely negative kind, of agents of at 
least two fundamentally different kinds. If we approach the series from 
one end, and watch the successive conquests of microscopical technique ; 
or if we consider the phenomena of immunity over the whole series; we 
are tempted to assume that all the viruses will ultimately be revealed as 
independent organisms. If we approach from the other end, or consider 
_ analogies from other examples of a transmissible alteration of metabolism, 
we may be tempted to doubt the significance of the evidence provided by 
the microscope, and to conclude that all viruses are unorganised, auto- 
genous, toxic principles. Hf we take the cautious attitude of supposing 
that both are right, and that viruses belonging to both these radically 
different types exist, where are we going to draw the line? Is the test 
to be one of unit dimension ? If so, what is the lower limit of the size 
of an organism? Are we to suppose that inclusion bodies can only be 
¢* & 
produced by viruses which are independent organisms? And if so, does 
this conclusion also apply to the ‘ X’ bodies associated with the infection 
of plant cells by certain viruses? If we try to form an estimate of the 
lower limit of size compatible with organisation, I think we should remember 
that particles which we measure by filters of known porosity, or by micro- 
photographs, need not be assumed to represent the virus organisms in an 
actively vegetative condition. They may well be minute structures, 
_ adapted to preserve the virus during transmission to cells in which it can 
resume vegetative life. Attempts to demonstrate an oxidative metabolism 
in extracts containing such a virus, separated from the cells in which it 
can grow and multiply, and to base conclusions as to the non-living nature 
_of the virus on failure to detect such activity, must surely be regarded as 
premature. Our evidence of the vitality of its particles is, as yet, entirely 
due to their behaviour after transmission. They may accordingly contain 
protein, lipoid and other molecules in a state of such dense aggregation, 
that comparisons of their size, with that of the heavily hydrated molecules 
of a protein in colloidal solution, may well give a misleading idea of their 
complexity. Workers in the cytology of genetics, accustomed to 
_ picturing a complex of potentialities as somehow packed into the compass 
of a gene, may find less difficulty, than does the bacteriologist, in 
attributing sufficient organisation, for true self-reproduction, even to 
particles still far beyond the range of detection by the microscope. If, in 
spite of such considerations, we find ourselves forced to the conclusion 
that some viruses consist of units so minute, that we cannot believe them 
to be living organisms, I hope we may avoid one common method of 
expressing the alternative conception, which refers to them as ‘ enzymes.’ 
I kmow of no evidence that any enzyme has the properties of a virus, or 
that any virus has those of an enzyme. We may regard it as a trans- 
missible toxin, when it causes the infected cell to disintegrate, or as a 
transmissible stimulant, when it induces an abnormal proliferation. In 
either case we must then postulate, as the special characteristic which 
makes it a virus, a power of imposing on the cell which it infects an altered 
metabolism, which leads to its own reproduction. 
A 
LO 
