34 
ZOOLOGICAL GEOGRAPHY. 
[part III. 
immigrants. We know that small Passerine birds annually reach 
the Bermudas from America, and the Azores from Europe, the 
former travelling over 600, the latter over 1000 miles of ocean. 
These groups of islands are both situated in stormy seas, and the 
immigrants are so numerous that hardly any specific change in the 
resident birds has taken place. The Galapagos receive no such 
annual visitants ; hence, when by some rare accident a few indi- 
viduals of a species did arrive, they remained isolated, probably 
for thousands of generations, and became gradually modified 
through natural selection under completely new conditions of 
existence. Less rare and violent storms would suffice to carry 
some of these to other islands, and thus the archipelago would 
in time become stocked. It would appear probable, that those 
which have undergone most change were the earliest to arrive ; 
so that we might look upon the three peculiar genera of finches, 
and Gerthidea, the peculiar form of Ccerebida?, as among the most 
ancient inhabitants of the islands, since they have become so 
modified as to have apparently no near allies on the mainland. 
But other birds may have arrived nearly at the same time, and 
yet not have been much changed. A species of very wide 
range, already adapted to live under very varied conditions and 
to compete with varied forms of life, might not need to become 
modified so much as a bird of more restricted range, and more 
specialized constitution. And if, before any considerable change 
had been effected, a second immigration of the same species 
occurred, crossing the breed would tend to bring back the original 
type of form. While, therefore, we may be sure that birds like 
the finches, which are profoundly modified and adapted to the 
special conditions of the climate and vegetation, are among the 
most ancient of the colonists ; we cannot be sure that the less 
modified form of tyrant-flycatcher or mocking-thrush, or even 
the unchanged but cosmopolitan owl, were not of coeval date ; 
since even if the parent form on the continent has been changed, 
successive immigrations may have communicated the same 
change to the colonists. 
The reptiles are somewhat more difficult to account for. We 
know, however, that lizards have some means of dispersal over 
