whole they are most like the species of the corresponding classes
inhabiting South America. Why should this be so? On the hypothesis of
special creation there is no reason why they should not be more like the
species of Africa or Australia than like those of the nearest body of the
mainland. The explanation given by evolution is clear, simple, and
reasonable. It is that the characteristic island forms are the descendants
of immigrants which in greatest probability would be wanderers from the
neighboring continent and not from far distant lands. Reaching the
isolated area in question the natural factors of evolution would lead
their offspring of later generations to vary from the original parental
types, and so the peculiar Galapagos species would come into being. The
fact that the organisms living on the various islands of this group differ
somewhat in lesser details adds further justification for the evolutionary
interpretation, because it is not probable that all the islands would be
populated at the same time by similar stragglers from the mainland. 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
Achatinellidæ 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