include the vast ice-capped
land-mass of Greenland (2,170,000 square kilometers or 846,000 square
miles). New Guinea, the largest habitable island, has only one-fourth
the area of Arabia, the largest of the peninsulas.[803] Therefore, both
the advantages and disadvantages incident to a restricted area may be
expected to appear in an intensified degree in islands.

Peninsulas are morphologically transition forms between mainland and
islands; by slight geological changes one is converted into the other.
Great Britain was a peninsula at the end of the Tertiary period, before
subsidence and the erosion of Dover Channel combined to sever it from
the continent. It bears to-day in its flora and fauna the evidence of
its former broad connection with the mainland.[804] In Pliocene times,
Sicily and Sardinia were united by a land bridge with the Tunisian
projection of North Africa; and they too, in their animal and plant
life, reveal the old connection with the southern continent.[805]
Sometimes man himself for his own purposes converts a peninsula into an
island. Often he constructs a canal, like that at Kiel or Corinth, to
remove an isthmian obstruction to navigation; but occasionally he
transforms his peninsula into an island for the sake of greater
protection. William of Rubruquis tells us that in 1253 he found the neck
of the Crimea cut through by a ditch from sea to sea by the native
Comanians, who had taken refuge in the peninsula from the Tartar
invaders, and in this way had sought to make their asylum more
secure.[806]

The reverse process in nature is quite as common. The Shangtung
Peninsula rises like a mountainous island from the sea-like level of
alluvial plains about it, suggesting that remote time when the plains
were not yet deposited and an arm of the Yellow Sea covered the space
between Shangtung and the highlands of Shansi.[807] The deposition of
silt, aided often by slight local elevation of the coast, is constantly
tying continental islands to the mainland. The Echinades Archipelago off
the southwest coast of ancient Acarnania, opposite the mouth of the
Achelous River, Strabo tells us, was formerly farther from shore than
in his time, and was gradually being cemented to the mainland by
Achelous silt. Some islets had already been absorbed in the advancing
shoreline, and the same fate awaited others.[808] Farther up this western
coast of Greece, the island of Leukas has been converted into a
peninsula by a si