an animal grows by the ingestion of food. Even movement is hardly an
absolute distinction between the living and the not-living; for no
movement can be detected in quiescent seeds, which may lie dormant for
thousands of years; and on the other hand inorganic foams when brought
into contact with liquids of different composition display movements
that very closely simulate those of the living matter. Lastly,
irritability, though so notably characteristic of living matter, is
scarcely peculiar to it, for many inorganic substances seem almost as
definitely responsive to external stimulation. But in the matter of
_their origin_ there is a real and a most fundamental difference. All
living substance arises only from other substance already living. It
cannot arise from the not-living; or at least it never has done so since
the beginning of scientific observation, though on this point have been
concentrated the learning and the laboratory technique of thousands of
chemists and microscopists.

It may not be out of place to quote here from one of the classics
dealing with this subject,--words that are just as true to-day as when
first written nearly half a century ago:

"Let us place vividly in our imagination the picture of the two great
kingdoms of nature,--the inorganic and the organic,--as these now stand
in the light of the Law of Biogenesis. What essentially is involved in
saying that there is no spontaneous generation of life? It is meant that
the passage from the mineral world to the plant or animal world is
hermetically sealed on the mineral side. This inorganic world is staked
off from the living world by barriers that have never yet been crossed
from within. No change of substance, no modification of environment, no
chemistry, no electricity, nor any form of energy, nor any evolution,
can endow a single atom of the mineral world with the attribute of