n. As regards the lowest forms
of life, the world is divided, and has for a long time been divided,
into two parties, the one affirming that we have only to submit
absolutely dead matter to certain physical conditions, to evolve from
it living things; the other (without wishing to set bounds to the
power of matter) affirming that, in our day, life has never been found
to arise independently of pre-existing life. I belong to the party
which claims life as a derivative of life. The question has two
factors--the evidence, and the mind that judges of the evidence; and
it may be purely a mental set or bias on my part that causes me
throughout this long discussion, to see, on the one side, dubious
facts and defective logic, and on the other side firm reasoning and a
knowledge of what rigid experimental enquiry demands. But, judged of
practically, what, again, has the question of Spontaneous Generation
to do with us? Let us see. There are numerous diseases of men and
animals that are demonstrably the products of parasitic life, and such
diseases may take the most terrible epidemic forms, as in the case of
the silkworms of France, referred to at an earlier part of this
article. Now it is in the highest degree important to know whether
the parasites in question are spontaneously developed, or whether they
have been wafted from without to those afflicted with the disease. The
means of prevention, if not of cure, would be widely different in the
two cases.

But this is not all. Besides these universally admitted cases, there
is the broad theory, now broached and daily growing in strength and
clearness--daily, indeed, gaining more and more of assent from the
most successful workers and profound thinkers of the medical
profession itself--the theory, namely, that contagious disease,
generally, is of this parasitic character. Had I any cause to regret
having introduced this theory to your notice more than a year ago,
that regret should now be expressed. I would certainly renounce in
your presence whatever leaning towards the germ theory my words might
then have betrayed. But since the time referred to nothing has
occurred to shake my conviction of the truth of the theory. Let me
briefly state the grounds on which its supporters rely. From their
respective viruses you may plant typhoid fever, scarlatina, or
small-pox. What is the crop that arises from this husbandry? As
surely as a thistle rises from a thistle seed, as