at no maggots were bred when he
simply excluded the flies from access to the dead body by covering it
with wire gauze, but that the blow-flies swarmed on the gauze and
vainly laid their eggs on it! It was only gradually recognised that
birth by means of eggs or germs extruded from parental organisms of
the same history and character as their offspring is the explanation
of all such swarms of flies, worms, and even mushrooms and moulds as
had been formerly ascribed to a mysterious power of breeding these
organisms possessed by inanimate dirt and refuse.

In spite of this progress in knowledge the belief in "spontaneous
generation" of such excessively minute organisms as the bacteria and
yeasts was general until Theodore Schwann in 1836 performed with them
just the same experiment as Redi had performed with blow-flies in
1668. He showed that if a putrescible liquid (for instance, soup) were
boiled in a retort so as to destroy all germs, and then the open neck
of the retort was kept heated in a flame, so that no floating germs
could enter alive, the soup did not putrefy, and no bacteria or other
organisms appeared in it. The old notions, nevertheless, survive to
this day. Peasants, fisher-folk, and even uneducated wealthy
countrymen cling to them with the confidence arising from profound
ignorance. And occasionally a man of some scientific training and
knowledge astonishes the world by a futile attempt to show that the
old fancies were true in regard, at any rate, to the lowest
microscopic forms of life. But these are but the echoes of the past;
we do not believe nowadays in "spontaneous generation," nor in sudden
transformations of lower into higher forms of life. The doctrine,
"_omne vivum e vivo_"--every living thing (in the present condition of
our earth) is born from a living thing--is now held by scientific
investigators as a reasonable generalisation of experience.

On the other hand, Harvey's dictum, "Every living thing comes from an
egg," is only true in a limited sense, namely, that whilst the
individual among most larger animals and plants is always traceable to
an egg-cell detached from a parental individual of a like kind of
species, there are whole groups and series of lower animals and most
plants in which the individual born or "developed" from an egg-cell
does not proceed when grown to full size to reproduce in turn by eggs
and fertilising sperms, but divides into two or more individuals or
gives off det