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's theory is that it only shifts the problem of
the origin of organisms (i.e. living creatures) from the earth to
elsewhere.
The third answer is that living creatures of a very simple sort may have
emerged on the earth's surface from not-living material, e.g. from some
semi-fluid carbon compounds activated by ferments. The tenability of
this view is suggested by the achievements of the synthetic chemists,
who are able artificially to build up substances such as oxalic acid,
indigo, salicylic acid, caffeine, and grape-sugar. We do not know,
indeed, what in Nature's laboratory would take the place of the clever
synthetic chemist, but there seems to be a tendency to complexity.
Corpuscles form atoms, atoms form molecules, small molecules large
ones.
Various concrete suggestions have been made in regard to the possible
origin of living matter, which will be dealt with in a later chapter. So
far as we know of what goes on to-day, there is no evidence of
spontaneous generation; organisms seem always to arise from pre-existing
organisms of the same kind; where any suggestion of the contrary has
been fancied, there have been flaws in the experimenting. But it is one
thing to accept the verdict "omne vivum e vivo" as a fact to which
experiment has not yet discovered an exception and another thing to
maintain that this must always have been true or must always remain
true.
If the synthetic chemists should go on surpassing themselves, if
substances like white of egg should be made artificially, and if we
should get more light on possible steps by which simple living creatures
may have arisen from not-living materials, this would not greatly affect
our general outlook on life, though it would increase our appreciation
of what is often libelled as "inert" matter. If the dust of the earth
did naturally give rise very long ago to living creatures, if they are
in a real sense born of her and of the sunshine, then the whole world
becomes more continuous and more vital, and all the inorganic groaning
and travailing becomes more intelligible.
§ 4