The Outline of Science, Vol. 1 (of 4)

J >> J. Arthur Thomson >> The Outline of Science, Vol. 1 (of 4)

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We are impressed by striking novelties that crop up--a clever dwarf, a
musical genius, a calculating boy, a cock with a 10 ft. tail, a
"wonder-horse" with a mane reaching to the ground, a tailless cat, a
white blackbird, a copper beech, a Greater Celandine with much cut up
leaves; but this sort of mutation is common, and smaller, less brusque
variations are commoner still. _They form the raw materials of possible
evolution._ We are actually standing before an apparently inexhaustible
fountain of change. This is evolution going on.

The Sporting Jellyfish

It is of interest to consider a common animal like the jellyfish
Aurelia. It is admirably suited for a leisurely life in the open sea,
where it swims about by contracting its saucer-shaped body, thus driving
water out from its concavity. By means of millions of stinging cells on
its four frilled lips and on its marginal tentacles it is able to
paralyse and lasso minute crustaceans and the like, which it then wafts
into its mouth. It has a very eventful life-history, for it has in its
early youth to pass through a fixed stage, fastened to rock or seaweed,
but it is a successful animal, well suited for its habitat, and
practically cosmopolitan in its distribution. It is certainly an
old-established creature. Yet it is very variable in colour and in size,
and even in internal structure. Very often it is the size of a saucer or
a soup-plate, but giants over two feet in diameter are well known. Much
more important, however, than variation in colour and size are the
inborn changes in structure. Normally a jellyfish has its parts in four
or multiples of four. Thus it has four frilled lips, four tufts of
digestive filaments in its stomach, and four brightly coloured
reproductive organs. It has eight sense-organs round the margin of its
disc, eight branched and eight unbranched radial canals running from the
central stomach to a canal round the circumference. The point of giving
these details is just this, that every now and then we find a jellyfish
with its parts in sixes, fives, or threes, and with a multitude of minor
idiosyncrasies. _Even in the well-established jellyfish there is a
fountain of change._

Sec. 1

Evolution of Plants

It is instructive to look at the various kinds of cabbages, such as
cauliflower and Brussels sprouts, kale and curly greens, and remember
that they are all scions of the not very promising wild cabbage found on
our shores. And are not all the aristocrat apple-trees of our orchards
descended from the plebeian crab-apple of the roadside? We know far too
little about the precise origin of our cultivated plants, but there is
no doubt that after man got a hold of them he took advantage of their
variability to establish race after race, say, of rose and
chrysanthemum, of potato and cereal. The evolution of cultivated plants
is continuing before our eyes, and the creations of Mr. Luther Burbank,
such as the stoneless plum and the primus berry, the spineless cactus
and the Shasta daisy, are merely striking instances of what is always
going on.

There is reason to believe that the domestic dog has risen three times,
from three distinct ancestors--a wolf, a jackal, and a coyote. So a
multiple pedigree must be allowed for in the case of the dog, and the
same is true in regard to some other domesticated animals. But the big
fact is the great variety of breeds that man has been able to fix, after
he once got started with a domesticated type. There are over 200
well-marked breeds of domestic pigeons, and there is very strong
evidence that all are descended from the wild rock-dove, just as the
numerous kinds of poultry are descended from the jungle-fowl of some
parts of India and the Malay Islands. Even more familiar is the way in
which man has, so to speak, unpacked the complex fur of the wild rabbit,
and established all the numerous colour-varieties which we see among
domestic rabbits. And apart from colour-varieties there are long-haired
Angoras and quaint lop-eared forms, and many more besides. All this
points to evolution going on.

The Romance of the Wheat

It is well-known that Neolithic man grew wheat, and some authorities
have put the date of the first wheat harvest at between fifteen thousand
and ten thousand years ago. The ancient civilisations of Babylonia,
Egypt, Crete, Greece, and Rome were largely based on wheat, and it is
highly probable that the first great wheatfields were in the fertile
land between the Tigris and the Euphrates. The oldest Egyptian tombs
that contain wheat, which, by the way, never germinates after its
millennia of rest, belong to the First Dynasty, and are about six
thousand years old. But there must have been a long history of wheat
before that.

Now it is a very interesting fact that the almost certain ancestor of
the cultivated wheat is at present living on the arid and rocky slopes
of Mount Hermon. It is called _Triticum hermonis_, and it is varying
notably to-day, as it did long ago when it gave rise to the emmer, which
was cultivated in the Neolithic Age and is the ancestor of all our
ordinary wheats. We must think of Neolithic man noticing the big seeds
of this Hermon grass, gathering some of the heads, breaking the brittle
spikelet-bearing axis in his fingers, knocking off the rough awns or
bruising the spikelets in his hand till the glumes or chaff separated
off and could be blown away, chewing a mouthful of the seeds--and
resolving to sow and sow again.

That was the beginning of a long story, in the course of which man took
advantage of the numerous variations that cropped up in this sporting
stock and established one successful race after another on his fields.
Virgil refers in the "Georgics" to the gathering of the largest and
fullest ears of wheat in order to get good seed for another sowing, but
it was not till the first quarter of the nineteenth century that the
great step was taken, by men like Patrick Sheriff of Haddington, of
deliberately selecting individual ears of great excellence and
segregating their progeny from mingling with mediocre stock. This is the
method which has been followed with remarkable success in modern times.

One of the factors that assisted the Allies in overcoming the food
crisis in the darkest period of the war was the virtue of Marquis Wheat,
a very prolific, early ripening, hard red spring wheat with excellent
milling and baking qualities. It is now the dominant spring wheat in
Canada and the United States, and it has enormously increased the real
wealth of the world in the last ten years (1921). Now our point is
simply that this Marquis Wheat is a fine example of evolution going on.
In 1917 upwards of 250,000,000 bushels of this wheat were raised in
North America, and in 1918 upwards of 300,000,000 bushels; yet the whole
originated from a single grain planted in an experimental plot at Ottawa
by Dr. Charles E. Saunders so recently as the spring of 1903.

[Illustration: THE WALKING-FISH OR MUD-SKIPPER (PERIOPHTHALMUS), COMMON
AT THE MOUTHS OF RIVERS IN TROPICAL AFRICA, ASIA, AND NORTH-WEST
AUSTRALIA

It skips about by means of its strong pectoral fins on the mud-flats; it
jumps from stone to stone hunting small shore-animals; it climbs up the
roots of the mangrove-trees. The close-set eyes protrude greatly and are
very mobile. The tail seems to help in respiration.]

[Illustration: _Photo: "The Times."_

THE AUSTRALIAN MORE-PORK OR PODARGUS

A bird with a frog-like mouth, allied to the British Nightjar. Now in
the London Zoological Gardens.

The capacious mouth is well suited for engulfing large insects such as
locusts and mantises, which are mostly caught on the trees. During the
day the More-pork or Frog-mouth sleeps upright on a branch, and its
mottled brown plumage makes it almost invisible.]

[Illustration: PELICAN'S BILL, ADAPTED FOR CATCHING AND STORING FISHES

There is an enormous dilatable sac beneath the lower jaw.]

[Illustration: HORNBILL'S BILL, ADAPTED FOR EXCAVATING A NEST IN A TREE,
AND ALSO FOR SEIZING AND BREAKING DIVERSE FORMS OF FOOD, FROM MAMMALS TO
TORTOISES, FROM ROOTS TO FRUITS

The use of the helmet or casque is obscure.]

[Illustration: SPOONBILL'S BILL, ADAPTED FOR SIFTING THE MUD AND
CATCHING THE SMALL ANIMALS, E.G. FISHES, CRUSTACEANS, INSECT LARVAE,
WHICH LIVE THERE]

[Illustration: FALCON'S BILL, ADAPTED FOR SEIZING, KILLING, AND TEARING
SMALL MAMMALS AND BIRDS]

[Illustration: AVOCET'S BILL, ADAPTED FOR A CURIOUS SIDEWAYS SCOOPING IN
THE SHORE-POOLS AND CATCHING SMALL ANIMALS]

[Illustration: PUFFIN'S BILL, ADAPTED FOR CATCHING SMALL FISHES NEAR THE
SURFACE OF THE SEA, AND FOR HOLDING THEM WHEN CAUGHT AND CARRYING THEM
TO THE NEST

The scaly covering is moulted in the autumn.]

We must not dwell too long on this particular instance of evolution,
though it has meant much to our race. We wish, however, following
Professor Buller's _Essays on Wheat_ (1919), to explain the method by
which this good seed was discovered. From one we may learn all. The
parent of Marquis Wheat on the male side was the mid-Europe Red Fife--a
first-class cereal. The parent on the female side was less promising, a
rather nondescript, not pure-bred wheat, called Red Calcutta, which was
imported from India into Canada about thirty years ago. The father was
part of a cargo that came from the Baltic to Glasgow, and was happily
included in a sample sent on to David Fife in Ontario about 1842. From
one kernel of this sample David Fife started his stock of Red Fife,
which was crossed by Dr. Saunders with Hard Red Calcutta. The result of
the cross was a medley of types, nearly a hundred varieties altogether,
and it was in scrutinising these that Dr. Saunders hit upon Marquis. He
worked steadily through the material, studying head after head of what
resulted from sowing, and selecting out those that gave most promise.
Each of the heads selected was propagated; most of the results were
rejected; the elect were sifted again and yet again, and finally Marquis
Wheat emerged, rich in constructive possibilities, probably the most
valuable food-plant in the world. It is like a romance to read that "the
first crop of the wheat that was destined within a dozen years to
overtax the mightiest elevators in the land was stored away in the
winter of 1904-5 in a paper packet no larger than an envelope."

Thus from the Wild Wheat of Mount Hermon there evolved one of the most
important food-plants of the world. This surely is _Evolution going on_.

Sec. 2

Changes in the Animal Life of a Country

Nothing gives us a more convincing impression of evolution in being than
a succession of pictures of the animal life of a country in different
ages. Dr. James Ritchie, a naturalist of distinction, has written a
masterly book, _The Influence of Man on Animal Life in Scotland_ (1920),
in which we get this succession of pictures. "Within itself," he says,
"a fauna is in a constant state of uneasy restlessness, an assemblage of
creatures which in its parts ebbs and flows as one local influence or
another plays upon it." There are temporary and local changes, endless
disturbances and readjustments of the "balance of nature." One year
there is a plague of field-voles, perhaps next year "grouse disease" is
rife; in one place there is huge increase of starlings, in another place
of rabbits; here cockchafers are in the ascendant, and there the moles
are spoiling the pasture. "But while the parts fluctuate, the fauna as a
whole follows a path of its own. As well as internal tides which swing
to and fro about an average level, there is a drift which carries the
fauna bodily along an 'irretraceable course.'" This is partly due to
considerable changes of climate, for climate calls the tune to which
living creatures dance, but it is also due to new departures among the
animals themselves. We need not go back to the extinct animals and lost
faunas of past ages--for Britain has plenty of relics of these--which
"illustrate the reality of the faunal drift," but it may be very useful,
in illustration of evolution in being, to notice what has happened in
Scotland since the end of the Great Ice Age.

Some nine thousand years ago or more, certain long-headed,
square-jawed, short-limbed, but agile hunters and fishermen, whom we
call Neolithic Man, established themselves in Scotland. What was the
state of the country then?

It was a country of swamps, low forests of birch, alder, and willow,
fertile meadows, and snow-capped mountains. Its estuaries penetrated
further inland than they now do, and the sea stood at the level of
the Fifty-Foot Beach. On its plains and in its forests roamed many
creatures which are strange to the fauna of to-day--the Elk and the
Reindeer, Wild Cattle, the Wild Boar and perhaps Wild Horses, a
fauna of large animals which paid toll to the European Lynx, the
Brown Bear and the Wolf. In all likelihood, the marshes resounded to
the boom of the Bittern and the plains to the breeding calls of the
Crane and the Great Bustard.

Such is Dr. Ritchie's initial picture.

[Illustration: LIFE-HISTORY OF A FROG

1, Before hatching; 2, newly hatched larvae hanging on to water-weed; 3,
with external gills; 4, external gills are covered over and are
absorbed; 5, limbless larva about a month old with internal gills; 6,
tadpole with hind-legs, about two months old; 7, with the fore-limbs
emerging; 8, with all four legs free; 9, a young frog, about three
months old, showing the almost complete absorption of the tail and the
change of the tadpole mouth into a frog mouth.]

[Illustration: _Photo: J. J. Ward. F.E.S._

HIND-LEG OF WHIRLIGIG BEETLE WHICH HAS BECOME BEAUTIFULLY MODIFIED FOR
AQUATIC LOCOMOTION

The flattened tips form an expanding "fan" or paddle, which opens and
closes with astonishing rapidity. The closing of the "fan," like the
"feathering" of an oar, reduces friction when the leg is being moved
forwards for the next stroke.]

[Illustration: THE BIG ROBBER-CRAB (_Birgus Latro_), THAT CLIMBS THE
COCO-NUT PALM AND BREAKS OFF THE NUTS

It occurs on islands in the Indian Ocean and Pacific, and is often found
far above sea-level. It is able to breathe dry air. One is seen emerging
from its burrow, which is often lined with coco-nut fibre. The empty
coco-nut shell is sometimes used by the Robber-Crab for the protection
of its tail.]

Now what happened in this kingdom of Caledonia which Neolithic Man had
found? He began to introduce domesticated animals, and that meant a
thinning of the ranks of predacious creatures. "Safety first" was the
dangerous motto in obedience to which man exterminated the lynx, the
brown bear, and the wolf. Other creatures, such as the great auk, were
destroyed for food, and others like the marten for their furs. Small
pests were destroyed to protect the beginnings of agriculture; larger
animals like the boar were hunted out of existence; others, like the
pearl-bearing river-mussels, yielded to subtler demands. No doubt there
was protection also--protection for sport, for utility, for aesthetic
reasons, and because of humane sentiments; even wholesome superstitions
have safeguarded the robin redbreast and the wren. There were
introductions too--the rabbit for utility, the pheasant for sport, and
the peacock for amenity. And every introduction, every protection, every
killing out had its far-reaching influences.

But if we are to picture the evolution going on, we must think also of
man's indirect interference with animal life. He destroyed the forests,
he cultivated the wild, he made bridges, he allowed aliens, like rats
and cockroaches, to get in unawares. Of course, he often did good, as
when he drained swamps and got rid of the mosquitoes which once made
malaria rife in Scotland.

What has been the net result? Not, as one might think for a moment, a
reduction in the _number_ of different kinds of animals. Fourteen or so
species of birds and beasts have been banished from Scotland since man
interfered, but as far as numbers go they have been more than replaced
by deliberate introductions like fallow deer, rabbit, squirrel, and
pheasant, and by accidental introductions like rats and cockroaches. But
the change is rather in _quality_ than in quantity; the smaller have
taken the place of the larger, rather paltry pigmies of noble giants.
Thus we get a vivid idea that evolution, especially when man interferes,
is not necessarily progressive. That depends on the nature of the sieves
with which the living materials are sifted. As Dr. Ritchie well says,
the standard of the wild fauna as regards size has fallen and is
falling, and it is not in size only that there is loss, there is a
deterioration of quality. "For how can the increase of Rabbits and
Sparrows and Earthworms and Caterpillars, and the addition of millions
of Rats and Cochroaches and Crickets and Bugs, ever take the place of
those fine creatures round the memories of which the glamour of
Scotland's past still plays--the Reindeer and the Elk, the Wolf, the
Brown Bear, the Lynx, and the Beaver, the Bustard, the Crane, the
Bumbling Bittern, and many another, lost or disappearing." Thus we see
again that evolution is going on.

Sec. 3

The Adventurers

All through the millions of years during which animals have tenanted the
earth and the waters under the earth, there has been a search for new
kingdoms to conquer, for new corners in which to make a home. And this
still goes on. _It has been and is one of the methods of evolution to
fill every niche of opportunity._ There is a spider that lives inside a
pitcher-plant, catching some of the inquisitive insects which slip down
the treacherous internal surface of the trap. There is another that
makes its home in crevices among the rocks on the shore of the
Mediterranean, or even in empty tubular shells, keeping the water out,
more or less successfully, by spinning threads of silk across the
entrance to its retreat. The beautiful brine-shrimp, _Artemia salina_,
that used to occur in British salterns has found a home in the dense
waters of the Great Salt Lake of Utah. Several kinds of earthworms have
been found up trees, and there is a fish, Arges, that climbs on the
stones of steep mountain torrents of the Andes. The intrepid explorers
of the _Scotia_ voyage found quite a number of Arctic terns spending our
winter within the summer of the Antarctic Circle--which means girdling
the globe from pole to pole; and every now and then there are incursions
of rare birds, like Pallas's Sand-grouse, into Britain, just as if they
were prospecting in search of a promised land. Twice or thrice the
distinctively North American Killdeer Plover has been found in Britain,
having somehow or other got across the Atlantic. We miss part of the
meaning of evolution if we do not catch this note of insurgence and
adventure, which some animal or other never ceases to sound, though many
establish themselves in a security not easily disturbed, and though a
small minority give up the struggle against the stream and are content
to acquiesce, as parasites or rottenness eaters, in a drifting life of
ease.

More important than very peculiar cases is the broad fact that over and
over again in different groups of animals there have been attempts to
master different kinds of haunts--such as the underground world, the
trees, the freshwaters, and the air. There are burrowing amphibians,
burrowing reptiles, burrowing birds, and burrowing mammals; there are
tree-toads, tree-snakes, tree-lizards, tree-kangaroos, tree-sloths,
tree-shrews, tree-mice, tree-porcupines, and so on; enough of a list to
show, without mentioning birds, how many different kinds of animals
have entered upon an arboreal apprenticeship--an apprenticeship often
with far-reaching consequences. What the freeing of the hand from being
an organ of terrestrial support has meant in the evolution of monkeys is
a question that gives a spur to our imagination.

The Case of the Robber Crab

On some of the coral islands of the Indian and Pacific Oceans there
lives a land-crab, Birgus, which has learned to breathe on land. It
breathes dry air by means of curious blood-containing tufts in the upper
part of its gill-cavity, and it has also rudimentary gills. It is often
about a foot long, and it has very heavy great claws, especially on the
left-hand side. With this great claw it hammers on the "eye-hole" of a
coconut, from which it has torn off the fibrous husk. It hammers until a
hole is made by which it can get at the pulp. Part of the shell is
sometimes used as a protection for the soft abdomen--for the
robber-crab, as it is called, is an offshoot from the hermit-crab stock.
Every year this quaint explorer, which may go far up the hills and climb
the coco-palms, has to go back to the sea to spawn. The young ones are
hatched in the same state as in our common shore-crab. That is to say,
they are free-swimming larvae which pass through an open-water period
before they settle down on the shore, and eventually creep up on to dry
land. Just as open-water turtles lay their eggs on sandy shores, going
back to their old terrestrial haunt, so the robber-crab, which has
almost conquered the dry land, has to return to the seashore to breed.
There is a peculiar interest in the association of the robber-crab with
the coco-palm, for that tree is not a native of these coral islands, but
has been introduced, perhaps from Mexico, by the Polynesian mariners
before the discovery of America by Columbus. So the learning to deal
with coconuts is a recent achievement, and we are face to face with a
very good example of evolution going on.

[Illustration: EARLY LIFE-HISTORY OF THE SALMON

1. The fertilised egg, shed in the gravelly bed of the river.

2. The embryo within the egg, just before hatching. The embryo has been
constricted off from the yolk-laden portion of the egg.

3. The newly hatched salmon, or alevin, encumbered with its legacy of
yolk (Y.S.).

4 and 5. The larval salmon, still being nourished from the yolk-sac
(Y.S.), which is diminishing in size as the fish grows larger.

6. The salmon fry about six weeks old, with the yolk fully absorbed, so
that the young fish has now to feed for itself. The fry become parr,
which go to the sea as smolts, and return as grilse.

In all cases the small figures to the right indicate the natural size.]

[Illustration: THE SALMON LEAPING AT THE FALL IS A MOST FASCINATING
SPECTACLE

Again and again we see them jumping out of the seething foam beneath the
fall, casting themselves into the curtain of the down-rushing water,
only to be carried back by it into the depths whence they have risen.
One here and another there makes its effort good, touches the upper lip
of the cataract, gives a swift stroke of its tail, and rushes on towards
those upper reaches which are the immemorial spawning beds of its
race.]

The Story of the Salmon

In late autumn or in winter the salmon spawn in the rivers. The female
makes a shallow trough in the gravel by moving her tail from side to
side, and therein lays many eggs. The male, who is in attendance,
fertilises these with the milt, and then the female covers them deeply
with gravel. The process is repeated over and over again for a week or
more till all the eggs are shed. For three to four months the eggs
develop, and eventually there emerge the larvae or _alevins_, which lurk
among the pebbles. They cannot swim much, for they are encumbered by a
big legacy of yolk. In a few weeks, perhaps eight, the protruding bag of
yolk has disappeared and the _fry_, about an inch long, begin to move
about more actively and to fend for themselves. By the end of the year
they have grown to be rather trout-like _parr_, about four inches long.
In two years these are double that length. Usually in the second year,
but it may be earlier or later, the parr become silvery _smolts_, which
go out to sea, usually about the month of May. They feed on young
herring and the like and grow large and strong. When they are about
three and a half years old they come up the rivers as _grilse_ and may
spawn. Or they may pass through the whole grilse stage in the sea and
come up the rivers with all the characters of the full-grown fish. In
many cases the salmon spawn only once, and some (they are called _kelts_
after spawning) are so much exhausted by starting a new generation that
they die or fall a victim to otters and other enemies. In the case of
the salmon of the North Pacific (in the genus _Oncorhynchus_, not
_Salmo_) all the individuals die after spawning, none being able to
return to the sea. It must be remembered that full-grown salmon do not
as a rule feed in fresh water, though they may be unable to resist
snapping at the angler's strange creations. A very interesting fact is
that the salmon keeps as it were a diary of its movements, which vary a
good deal in different rivers. This diary is written in the scales, and
a careful reading of the concentric lines on the scales shows the age of
the fish, and when it went out to sea, and whether it has spawned or
not, and more besides.

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