therefore require a distinct origin, it makes little difference whether
we looked upon that origin as a single originating point or as thousands
of independent creations. But so soon as it appeared that the present
condition of the earth's crust was formed by the action of forces still
in existence, and so soon as it appeared that the forces outside of
living forces, including astronomical, physical and chemical forces, are
all correlated with each other as parts of the same store of energy,
then the problem of the origin of living things assumed a new meaning.
Living things became then a part of nature, and demanded to be included
in the same general category. The reign of law, which was claiming that
all nature's phenomena are the result of natural rather than
supernatural powers, demanded some explanation of the origin of living
things. Consequently, when Darwin pointed out a possible way in which
living phenomena could thus be included in the realm of natural law,
science was ready and anxious to receive his explanation.

==Cytology.==--A third conception which contributed to the formulation of
modern biology was derived from the facts discovered in connection with
the organic cell and protoplasm. The significance of these facts we
shall notice later, but here we may simply state that these discoveries
offered to students simplicity in the place of complexity. The doctrine
of cells and protoplasm appeared to offer to biologists no longer the
complicated problems which were associated with animals and plants, but
the same problems stripped of all side issues and reduced to their
lowest terms. This simplifying of the problems proved to be an
extraordinary stimulus to the students who were trying to find some way
of understanding life.

==New Aspects of Biology==.--These three conceptions seized hold of the
scientific world at periods not very distant from each other, and their