nd have been most frequently made. And they follow the course we have
just indicated; the "apparently" vitalistic and psychical behaviour of
unicellulars (impulse, will, spontaneous movement, selecting and
experimenting) is interpreted in terms of reflex processes and the
"irritability" of the cell, and these again are traced back, like all
stimulus-processes, to the subtle mechanics of the atoms.

Spontaneous Generation.

4. This reduction of known biological phenomena to simpler terms, the
lessening of the gap between inorganic and organic chemistry, and the
formulation of the doctrine of the conservation of energy, have all
prepared the way for a fourth step, the establishment of the inevitable
theory of _generatio spontanea sive equivoca_, the spontaneous generation
of the living, that is to say, the gradual evolution of the living from
the not living. Since the earth, and with it the conditions under which
alone life is possible, have had a beginning in time, life upon the earth
must also have had a beginning. The assumption that the first living
organisms may have come to the earth on meteorites simply shifts the
problem a step farther back, for according to all current theories of the
universe, if there are in any of the heavenly bodies conditions admitting
of the presence of life, these conditions have arisen from others in which
life was impossible. Therefore, since this suggestion is on the face of it
a mere evasion of the difficulty, the theory of spontaneous generation
naturally arose. There is something almost comical in the change in the
attitude of the natural sciences to this theory. For centuries it was one
of the beliefs of popular superstition, with its naive way of regarding
nature, that earthworms "developed" from damp soil, and vermin from
shavings, and in general that the living arose from the non-living. On the
other hand it was one of the characteristics and axioms of scientific
thought to reject this naive _generatio equivoca_, and to hold fast to the
proposition, _omne vivum ex ovo_, or, at least, _omne vivum ex vivo_. And
it was regarded as one of the triumphs of modern science when, about the
middle of the last century, Pasteur gave definiteness to this doctrine,
and when through him, through Virchow, and indeed the whole younger
generation of naturalists, the proposition was modified, on the basis of
the newly discovered cell-theory, to _omnis cellula ex cellula_. But a
short time a