GEOGRAPHICAL DISTRIBUTION 407 



827. The fauna of isolated oceanic islands contributes the 

 same kind of evidence. Madagascar has many species found 

 nowhere else, but these species are more like those of the 

 neighboring African coast than those of more remote regions. 

 The Galapagos Islands also have a peculiar fauna, which finds 

 its greatest affinity in the fauna of the nearest part of the South 

 American coast. The fauna of the Azores is related to that of 

 Europe, while that of the Bermudas belongs to America. 

 These resemblances of faunas cannot be attributed to the effect 

 of food and climate or other external causes, for there is often 

 a greater difference in environment between two neighboring 

 localities than between others on opposite sides of the globe. 



828. The Hawaiian Islands are very mountainous and the 

 mountains are cut by numerous deep parallel valleys. In 

 these valleys are found numerous species of the snail, Achati- 

 nella. The snails cannot cross the mountain barriers and hence 

 there is little migrating of snails from valley to valley. The 

 fact that almost every valley has its own peculiar type of 

 snail, and the way these species are distributed on the island, 

 makes it seem probable that all had a common origin and 

 that each species originated where it is now found. If at any 

 time, in any valley, a new character appears through individual 

 variation that character may in time be transmitted to all the 

 individuals of that valley, and hence become a specific char- 

 acter, but natural barriers will prevent its transmission to other 

 species living in other valleys. 



ADAPTATIONS 



829. In Part I many examples of modification of the type 

 structures were described and it was shown that these modifi- 

 cations were always associated with peculiarities of life habit 

 or of environment. Such modifications of an organism, in 

 connection with peculiarities of the condition of life, are known 



