fter Pasteur's discoveries, the ideas of Darwinism and the
theory of evolution gained widespread acceptance. And now it appeared
that, in rejecting the theory of _generatio equivoca_, naturalists had, so
to speak, sawn off the branch on which they desired to sit, and thus many,
like Haeckel, became enthusiastic converts to the theory which natural
science had previously rejected.

Constructing theories and speculations as to the possibilities of
spontaneous generation is regarded by some naturalists as somewhat
gratuitous (_cf._ Du Bois-Reymond). In general, it is regarded as
sufficient to point out that the reduction of the phenomena of life as we
know them to those of a simpler order, and the unification of organic and
inorganic chemistry, have made the problem of the first origin of life
essentially simpler, and that the law of the constancy and identity of
energy throughout the universe permits no other theory. But others go more
determinedly to work, and attempt to give concrete illustrations of the
problem. The most elementary form of life known to us is the cell. From
cells and their combinations, their products and secretions, all
organisms, plant and animal alike, are built up. If we succeed in deriving
the cell, the derivation of the whole world of life seems, with the help
of the doctrine of descent, a comparatively simple matter. The cell itself
seems to stand nearer to the inorganic, and to be less absolutely apart
from the inanimate world than a highly organised body, differentiated as
to its functions and organs, such as a mammal. It almost seems as if we
might regard the lowest forms of life known to us, which seem little more
than aggregated homogeneous masses of flowing rather than creeping
protoplasm, as an intermediate link between the higher forms of life and
the non-living. But the theory does not begin with the cell; it assumes a
series of connecting-links (which may of course be as long and as
complicated as the series from the cell upwards to man) between the cell
and matter which is still quite "inorganic" and which is capable only of
the everyday chemical and physical phenomena, and not of the higher
syntheses of these, which in their increasing complexity and diversity
ultimately come to represent "life" in its most primitive forms. As
proteid is the chief constituent of protoplasm, it is regarded as the
specific physical basis of life, and life is looked upon as the sum of its
functions. And