34 ZOOLOGICAL GEOGRAPHY. [part hi. 



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 Certhidea, the peculiar form of Ccerebidse, 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 



