When we examine carefully the differences between the living and the
not-living, we see that the chief difference between them is in _their
origin_. The matter of growth is not a real distinction; for crystals
grow on the outside, while inorganic liquids grow by intussusception, as
when a soluble substance is added to them, in very much the same way as
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.
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