of the terms positive and negative.
[Footnote: See his truly philosophical remarks on this head in the
'British Medical Journal,' 1876, p. 282.] Not such, I trust, would
be the course pursued by my friend. As regards the 50 fruitful flasks
he would, I doubt not, repeat the experiment with redoubled care and
scrutiny, and not by one repetition only, but by many, assure himself
that he had not fallen into error. Such faithful scrutiny fully
carried out would infallibly lead him to the conclusion that here, as
in all other cases, the evidence in favour of spontaneous generation
crumbles in the grasp of the competent enquirer.

The botanist knows that different seeds possess different powers of
resistance to heat. [Footnote: I am indebted to Dr. Thiselton Dyer for
various illustrations of such differences. It is, however, surprising
that a subject of such high scientific importance should not have been
more thoroughly explored. Here the scoundrels who deal in killed
seeds might be able to add to our knowledge.] Some are killed by a
momentary exposure to the boiling temperature, while others withstand
it for several hours. Most of our ordinary seeds are rapidly killed,
while Pouchet made known to the Paris Academy of Sciences in 1866,
that certain seeds, which had been transported in fleeces of wool from
Brazil, germinated after four hours' boiling. The germs of the air
vary as much among themselves as the seeds of the botanist. In some
localities the diffused germs are so tender that boiling for five
minutes, or even less, would be sure to destroy them all; in other
localities the diffused germs are so obstinate, that many hours'
boiling would be requisite to deprive them of their power of
germination. The absence or presence of a truss of desiccated hay
would produce differences as great as those here described. The
greatest endurance that I have ever observed--and I believe it is the
greatest on record--was a case of survival after eight hours' boiling.

As regards their power of resisting heat, the infusorial germs of our
atmosphere might be classified under the following and intermediate
heads: Killed in five minutes; not killed in five minutes but killed
in fifteen; not killed in fifteen minutes but killed in thirty; not
killed in thirty minutes but killed in an hour; not killed in an hour
but killed in two hours; not killed in two but killed in three hours;
not killed in three but killed in four hours. I h