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K5MOTE islands have a fascination for the biologist. Their in-
accessibility makes them a sanctuary, both for rare species and for
immense congregations of commoner ones. Their isolation has pre-
vented many forms from reaching them at all, so that what they lack
is as interesting as what they possess. And this same isolation, com-
bined with the difference of conditions, has often encouraged the
evolution of special local types.
St. Kilda has all these biological attractions. It is scientifically
celebrated as the home of the St. Kilda wren, a subspecies of the
common wren so distinct that it was for some time classified as a
separate species. It is one of the few places in Britain where Leach's
fork-tailed.petrel nestsa beautiful little creature still more martin-
like than its common relative the storm petrel. On one of its three
main component islands there lives over a fifth of the world's entire
stock of gannetsthose most spectacular of all our sea-birds; while
a conservative estimate of its puffin population would be a quarter
of a million. It has a melancholy historical attraction as the site of
the last recorded British occurrence of the great auk. In 1821, only
twenty-three years before the final extinction of the species, a speci-
men was captured there on a ledge of cliff. It eventually passed into
the hands of John Fleming, who kept it alive for some time on the
vessel of the Northern Lighthouse Commissioners. Unfortunately,
when they reached the Clyde, the bird escaped while being indulged
with a swim in the sea, contriving to slip the cord attached to
one leg.
St. Kilda also forms a part of a region where evolution can be
studied in action. All round the north-west and north of Scotland,
the islands harbour animals and plants which are slightly different
from those of the mainland. To take but a few examples from birds,
the Shetland wren is also distinct enough to be classified as a distinct
subspecies. So is the Shetland starling, and the hedge-sparrow and
the song-thrush from the Hebrides.
What is more, the distinctive types of the Scottish islands form part
of a graded system, a field of change, which extends inwards to the
mainland coasts and outwards to the Faeroes and Iceland. If you
take measurements of the different local races of wrens, you find that
they increase in size at a pretty definite rate with increasing north
latitudealmost i  per cent, increase in size for every degree. The
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