GEOGRAPHICAL DISTRIBUTION OF ANIMALS 279 



The Galapagos Islands are a cluster of volcanic rocks 

 lying in the open sea about six hundred miles to the west 

 of Ecuador. On these islands is a peculiar land fauna, de- 

 rived from South American stock, but mostly different in 

 species. Darwin noted there "twenty-six land birds; of 

 these, twenty-one, or perhaps twenty-three, are ranked as 

 distinct species. Yet the close affinity of most of these 

 birds to American species is manifest in every character, in 

 their habits, gestures, and tones of voice. 7 ' 



Among land animals similar migrations may occur, giv- 

 ing rise, through the adaptation to new conditions, to new 

 species. The separation of species of animals isolated in 

 river basins or lakes often permits the acquisition of new 

 characters, which is the formation of distinct species in 

 similar fashion. On the west side of Mount Whitney, the 

 highest mountain in the Sierra Nevada of California, there 

 is a little stream called Volcano Creek. In this brook is a 

 distinct species or form of trout, locally called golden 

 trout. It is unusually small, very brilliantly colored, its 

 fins being bright golden, and its tiny scales scarcely over- 

 lap each other along its sides. This stream flows over a 

 high waterfall (Agua Bonita) into the Kern Eiver. The 

 Kern River is full of trout, of a kind (Salmo gilberti) to 

 which the golden trout is most closely allied. There can 

 not be much doubt that the latter is descended from the 

 former. With this assumption, it is easy to suppose that 

 once the waterfall did not exist, or that through some 

 agency we can not now identify certain fishes had been 

 carried over it. Once above it, they can not now return, 

 nor can they mix with the common stock of the river. 

 Those best adapted to the little stream have survived. 

 The process of adaptation has gone on till at last a distinct 

 species (or sub-species *) is formed. In recent times the 



* In descriptive works the name species is applied to a form when 

 the process of adaptation seems complete. When it is incomplete, or 



