upon the Earth

During the early chapters of the earth's history, no living creature
that we can imagine could possibly have lived there. The temperature was
too high; there was neither atmosphere nor surface water. Therefore it
follows that at some uncertain, but inconceivably distant date, living
creatures appeared upon the earth. No one knows how, but it is
interesting to consider possibilities.

[Illustration: _Reproduced from the Smithsonian Report, 1915._

A LIMESTONE CANYON

Many fossils of extinct animals have been found in such rock
formations.]

[Illustration: GENEALOGICAL TREE OF ANIMALS

Showing in order of evolution the general relations of the chief classes
into which the world of living things is divided. This scheme represents
the present stage of our knowledge, but is admittedly provisional.]

[Illustration: DIAGRAM OF AMOEBA

(Greatly magnified.)

The amoeba is one of the simplest of all animals, and gives us a hint
of the original ancestors. It looks like a tiny irregular speck of
greyish jelly, about 1/100th of an inch in diameter. It is commonly
found gliding on the mud or weeds in ponds, where it engulfs its
microscopic food by means of out-flowing lobes (PS). The food vacuole
(FV) contains ingested food. From the contractile vacuole (CV) the waste
matter is discharged. N is the nucleus, GR, granules.]

From ancient times it has been a favourite answer that the dust of the
earth may have become living in a way which is outside scientific
description. This answer forecloses the question, and it is far too soon
to do that. Science must often say "Ignoramus": Science should be slow
to say "Ignorabimus."

A second position held by Helmholtz, Lord Kelvin, and others, suggests
that minute living creatures may have come to the earth from elsewhere,
in the cracks of a meteorite or among cosmic dust. It must be remembered
that seeds can survive prolonged exposure to very low temperatures; that
spores of bacteria can survive high temperature; that seeds of plants
and germs of animals in a state of "latent life" can survive prolonged
drought and absence of oxygen. It is possible, according to Berthelot,
that as long as there is not molecular disintegration vital activities
may be suspended for a time, and may afterwards recommence when
appropriate conditions are restored. Therefore, one should be slow to
say that a long journey through space is impossible. The obvious
limitation of Lord Kelvin'