The
first settlers in one place would send out colonies to others, where
independent evolution would result in the appearance of minor differences
peculiar to the single island. In this manner science interprets the
general agreement between the animals of the Azores Islands and the fauna
of the northwestern part of Africa, the nearest body of land, from which
it would be most natural for the ancestors of the island fauna to come.
The land-snails inhabiting the various groups of islands scattered
throughout the vast extent of the Pacific Ocean provide the richest and
most ideal material for the demonstration of the principles of
geographical distribution. In the Hawaiian Islands snails of the family of
Achatinellidae occur in great abundance, and like the lizards of the
Galapagos Islands different species occur on the different members of the
group. Within the confines of one and the same island, they vary from
valley to valley, and the correlation between their isolation in
geographical respects and specific differences on the other hand, first
pointed out by Gulick, makes this tribe of animals classical material. In
Polynesia and Melanesia are found close relatives of the Achatinellidae,
namely, the Partulae, which are thus in relative proximity to the
Achatinellidae and not on the other side of the world.
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