178 SECTIONAL ADDRESSES. 
infected by it, we should be driven, I think, to the hypothesis which has 
been freely used to account for the propagation of bacteriophage, on the 
one hand, and of typical viruses like that of herpes, on the other ; namely, 
that the presence of the virus in a cell in some way constrains the metabolism 
of the cell to produce more. Bordet has used the reproduction of thrombin 
by the clotting of the blood, as an analogy for the suggested reproduction 
of bacteriophage in this manner. Another, and perhaps closer, analogy 
might be found in recent evidence that a culture of pneumococcus, 
deprived of its type-specific carbohydrate complex, can be made to take 
up the carbohydrate characteristic of another type, and then to reproduce 
itself indefinitely with this new, artificially imposed specificity. The 
response of the cells of the animal body, to even a single contact with a 
foreign protein, by the altered metabolism producing immunity, and 
often persistent for the lifetime of the individual, may suggest another 
parallel ; but here the protective type of the reaction is in direct contrast 
to the supposed regeneration by the cells of the poison which killed them. 
Boycott, again, has emphasised the difficulty of drawing a sharp line of dis- 
tinction between the action of normal cell-constituents, which promote 
cell-proliferation for normal repair of an injury, and the virus transmitting 
a malignant tumour, or that causing foot-and-mouth disease. I do not 
myself find it easy, on general biological grounds, to accept this idea of a 
cell having its metabolism thus immediately diverted to producing the 
agent of its own destruction, or abnormal stimulation. It is almost the 
direct opposite of the immunity reaction, which is not absent, but peculiarly 
effective in the response of the body to many viruses. It is difficult, again, 
to imagine that a virus like rabies could be permanently excluded from a 
country if it had such an autogenous origin. The phenomena of immunity 
to a virus, and of closely specific immunity to different strains of the same 
virus, are peculiarly difficult to interpret on these lines. This conception, 
however, of the reproduction of a virus by the perverted metabolism oi 
the infected cell, has been strongly supported by Doerr, in explanation of 
the phenomena of herpes. There are individuals in whom the epidermal 
cells have acquired a tendency to become affected by an herpetic eruption, 
in response to various kinds of systemic or local injury. From the lesions 
so developed, an agent having the typical properties of‘a virus can be 
obtained, capable of reproducing the disease by inoculation into individuals, 
even of other species, such as the rabbit, and exciting, when appropriately 
injected, the production of an antiserum, specifically antagonising the 
herpes infection. Such phenomena have a special interest for our dis- 
cussion, in that they can be almost equally well explained by the two rival 
conceptions. One regards the herpes virus as a distinct ultramicroscopic 
organism, and the person liable to attack as a carrier, in whom the virus 
can be awakened to pathogenic activity and multiplication, by injuries 
weakening the normal resistance of his cells to mvasion. The other 
regards it as a pathogenic principle produced by the cells in response to 
injury, and awakening other cells to further production when it is trans- 
mitted to them. 
This forms a good example of the central difficulty which we should 
try to solve in to-day’s discussion, on the group of agents at present classed 
together as viruses. They seem to form a series; but we do not know 
a a i 
