However it may have come about, there is no doubt at all that one of the
first great steps in Organic Evolution was the forking of the
genealogical tree into Plants and Animals--the most important parting of
the ways in the whole history of Nature.
Typical plants have chlorophyll; they are able to feed at a low chemical
level on air, water, and salts, using the energy of the sunlight in
their photosynthesis. They have their cells boxed in by cellulose walls,
so that their opportunities for motility are greatly restricted. They
manufacture much more nutritive material than they need, and live far
below their income. They have no ready way of getting rid of any
nitrogenous waste matter that they may form, and this probably helps to
keep them sluggish.
Animals, on the other hand, feed at a high chemical level, on the
carbohydrates (e.g. starch and sugar), fats, and proteins (e.g. gluten,
albumin, casein) which are manufactured by other animals, or to begin
with, by plants. Their cells have not cellulose walls, nor in most cases
much wall of any kind, and motility in the majority is unrestricted.
Animals live much more nearly up to their income. If we could make for
an animal and a plant of equal weight two fractions showing the ratio of
the upbuilding, constructive, chemical processes to the down-breaking,
disruptive, chemical processes that go on in their respective bodies,
the ratio for the plant would be much greater than the corresponding
ratio for the animal. In other words, animals take the munitions which
plants laboriously manufacture and explode them in locomotion and
work; and the entire system of animate nature depends upon the
photosynthesis that goes on in green plants.
[Illustration: _From the Smithsonian Report, 1917_
A PIECE OF A REEF-BUILDING CORAL, BUILT UP BY A LARGE COLONY OF SMALL
SEA-ANEMONE-LIKE POLYPS, EACH OF WHICH FORMS FROM THE SALTS OF THE SEA A
SKELETON OR SHELL OF LIME
The wonderful mass of corals, which are very beautiful, are the skeleton
remains of hundreds of these little creatures.]
[Illustration: _Photo: J. J. Ward, F.E.S._
THE INSET CIRCLE SHOWS A GROUP OF CHALK-FORMING ANIMALS, OR
FORAMINIFERA, EACH ABOUT THE SIZE OF A VERY SMALL PIN'S HEAD
They form a great part of the chalk cliffs of Dover and similar deposits
which have been raised from the floor of an ancient sea.
THE ENORMOUSLY ENLARGED ILLUSTRATION IS THAT OF A COMMON FORAMINIFER
(POLYSTOMELLA) SHOWING THE SHELL IN THE CENTRE AND THE OUTFLOWING
NETWORK OF LIVING MATTER, ALONG WHICH GRANULES ARE CONTINUALLY
TRAVELLING, AND BY WHICH FOOD PARTICLES ARE ENTANGLED AND DRAWN IN
_Reproduced by permission of the Natural History Museum_ (_after Max
Schultze_).]
As the result of much more explosive life, animals have to deal with
much in the way of nitrogenous waste products, the ashes of the living
fire, but these are usually got rid of very effectively, e.g. in the
kidney filters, and do not clog the system by being deposited as
crystals and the like, as happens in plants. Sluggish animals like
sea-squirts which have no kidneys are exceptions that prove the rule,
and it need hardly be said that the statements that have been made in
regard to the contrasts between plants and animals are general
statements. There is often a good deal of the plant about the animal, as
in sedentary sponges, zoophytes, corals, and sea-squirts, and there is
often a little of the animal about the plant, as we see in the movements
of all shoots and roots and leaves, and occasionally in the parts of the
flower. But the important fact is that on the early forking of the
genealogical tree, i.e. the divergence of plants and animals, there
depended and depends all the higher life of the animal kingdom, not to
speak of mankind. The continuance of civilisation, the upkeep of the
human and animal population of the globe, and even the supply of oxygen
to the air we breathe, depend on the silent laboratories of the green
leaves, which are able with the help of the sunlight to use carbonic
acid, water, and salts to build up the bread of life.
§ 2