PRESIDENTIAL ADDRESS. ELS 
lish and develop pathogenic properties. It is well-known that 
repeated transmission of an organism from one host to an- 
other tends frequently to accentuate its pathogenic proper- 
ties, latent perhaps at first. Amongst the saprophytic spiro- 
chetes inhabiting the generative mucous membranes of 
man, one would expect that a mutant, possessing more than 
its fellows the capacity to invade living tissues, would have 
this property increased by repeated conveyance from one 
warm moist human mucous surface to another. Promiscuous 
sexual intercourse gave it this opportunity. The result we 
see to-day. 
A still further evolution in complexity of life-processes 
is seen in certain spirochetes. This consists in the con-. 
veyance of infection by intermediate hosts, ticks and bed- 
bugs for instance. How this has come about from sapro- 
phytic types is questionable. That these forms have arisen 
from such types in ages long ago is undoubted. The course 
of events was presumably one of two. The saprophyte, 
having become an invasive parasite, eventually reached the 
blood-stream, as it does now in syphilis. The arthropod, 
drawing this blood, ingested the spirochetes, which found 
themselves capable of multiplying in their new surround- 
ings. Thence they entered the tissues of a fresh vertebrate 
_ host when the arthropod fed again. The second alternative 
is that the pathogenic spirochetes were primarily arthro- 
pod dwellers (we have seen that they are found in ter- 
mites). The arthropod, in feeding on its host, either in- 
jected its spirochetes or fouled the wound it made with 
fecal matter containing these. In the new environment, the 
spirochetes were able to grow and multiply, and were not 
destroyed, whilst the vertebrate host reacted, in the shape 
of illness, to their presence. Bitten by further arthropods, 
the spirochetes entered again their original hosts. In either 
case, in the course of time, the spirochetes became more 
