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 larvæ 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 æsthetic
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.
§ 3