on-wool, will remain clear for an indefinite time. I have sought in
vain for any explanation of these facts, except the obvious one, that the
air contains germs competent to give rise to _Bacteria_, such as those
with which the first solution has been knowingly and purposely
inoculated, and to the mould-_Fungi_. And I have not yet been able to
meet with any advocate of Abiogenesis who seriously maintains that the
atoms of sugar, tartrate of ammonia, yeast-ash, and water, under no
influence but that of free access of air and the ordinary temperature,
re-arrange themselves and give rise to the protoplasm of _Bacterium_. But
the alternative is to admit that these _Bacteria_ arise from germs in the
air; and if they are thus propagated, the burden of proof that other like
forms are generated in a different manner, must rest with the assertor of
that proposition.

[Footnote 10: Infusion of hay treated in the same way yields similar
results; but as it contains organic matter, the argument which follows
cannot be based upon it.]

To sum up the effect of this long chain of evidence:--

It is demonstrable that a fluid eminently fit for the development of the
lowest forms of life, but which contains neither germs, nor any protein
compound, gives rise to living things in great abundance if it is exposed
to ordinary air; while no such development takes place, if the air with
which it is in contact is mechanically freed from the solid particles
which ordinarily float in it, and which may be made visible by
appropriate means.

It is demonstrable that the great majority of these particles are
destructible by heat, and that some of them are germs, or living
particles, capable of giving rise to the same forms of life as those
which appear when the fluid is exposed to unpurified air.

It is demonstrable that inoculation of the experimental fluid with a drop
of liquid known to contain living particles gives rise to the same
phenomena as exposure to unpurified air.

And it is further certain that these living particles are so minute that
the assumption of their suspension in ordinary air presents not the
slightest difficulty. On the contrary, considering their lightness and
the wide diffusion of the organisms which produce them, it is impossible
to conceive that they should not be suspended in the atmosphere in
myriads.

Thus the evidence, direct and indirect, in favour of _Biogenesis_ for all
known forms of life must, I think, be a