or by Xenogenesis, within the silkworm
or its moth, the extirpation of the disease must depend upon the
prevention of the occurrence of the conditions under which this
generation takes place. But if, on the other hand, the _Panhistophyton_
is an independent organism, which is no more generated by the silkworm
than the mistletoe is generated by the apple-tree or the oak on which it
grows, though it may need the silkworm for its development in the same
way as the mistletoe needs the tree, then the indications are totally
different. The sole thing to be done is to get rid of and keep away the
germs of the _Panhistophyton_. As might be imagined, from the course of
his previous investigations, M. Pasteur was led to believe that the
latter was the right theory; and, guided by that theory, he has devised a
method of extirpating the disease, which has proved to be completely
successful wherever it has been properly carried out.

There can be no reason, then, for doubting that, among insects,
contagious and infectious diseases, of great malignity, are caused by
minute organisms which are produced from pre-existing germs, or by
homogenesis; and there is no reason, that I know of, for believing that
what happens in insects may not take place in the highest animals.
Indeed, there is already strong evidence that some diseases of an
extremely malignant and fatal character to which man is subject, are as
much the work of minute organisms as is the Pebrine. I refer for this
evidence to the very striking facts adduced by Professor Lister in his
various well-known publications on the antiseptic method of treatment. It
appears to me impossible to rise from the perusal of those publications
without a strong conviction that the lamentable mortality which so
frequently dogs the footsteps of the most skilful operator, and those
deadly consequences of wounds and injuries which seem to haunt the very
walls of great hospitals, and are, even now, destroying more men than die
of bullet or bayonet, are due to the importation of minute organisms into
wounds, and their increase and multiplication; and that the surgeon who
saves most lives will be he who best works out the practical consequences
of the hypothesis of Redi.

I commenced this Address by asking you to follow me in an attempt to
trace the path which has been followed by a scientific idea, in its long
and slow progress from the position of a probable hypothesis to that of
an established la