fact--and continue these conditions during the proper period of gestation,
they _might_ produce life _de novo_.[13] But the most daring physicist
would stand aghast at the bare proposal of such an experiment. Neither his
knowledge of chemistry, nor the present uncertain value attaching to
"molecular machinery," would justify him, for a moment, in entering upon
such a purely tentative and empirical an undertaking.

It is hardly necessary to assume that the same law of vital force governs
in the appearance and geographical distribution of _fungi_, as universally
obtains in the higher and more complex vegetal growths. And although it
may be difficult, in some instances, to draw the precise line between
certain low mycological forms and the amoeboid and some other primitive
manifestations of animal life, yet all vegetable physiologists agree in
assigning a purely vegetable origin to all the primary groups of
fungi--their general cellular character determining their proper place in
classification. And in all their extended family groups, pervading nature
as widely as animal and vegetable life, we find that uniform chemical and
other conditions produce uniform mycological results. Spores are no more
necessary for their appearance, in the first instance, than acorns are
essential to the appearance of an oak forest when it succeeds the pine.
Wherever the necessary conditions of moisture and heat are found to
obtain, in connection with decayed or decaying substances, the particular
form of fungus indicated thereby, whether parasitic or non-parasitic, will
make its appearance. Continuously damp walls, or wall-paper, will produce
them in specific variety, not because their invisible spores are flying
about in the atmosphere to find appropriate lodgment, but because the
necessary conditions obtain for their manifestation, or for the
development of their vital units--those everywhere diffused, and ready to
burgeon forth from the proper matrix, or from certain nutrient conditions
to be met with in all vegetable substances, after the process of decay has