476 DISTRIBUTION OF LAND ANIMALS 



tically the same for a long period of time, the descendants 

 closely resemble the origuial, pioneer ancestors. But if 

 the conditions gradually change, and this is generally 

 the case, (c) the successful species gradually changes 

 also and so continues to find the territory favorable. 



It has frequently happened in former geological ages 

 that a few individuals of a species have been born which 

 differed somewhat from their parents, and this differ- 

 ence has been sufficient to enable them to penetrate to 

 some extent into new regions. Still further modified 

 descendants in succeeding generations have corre- 

 spondingly extended the range of the species, and by 

 this gradual process wide areas have been occupied. 

 Small groups of individuals within these large areas may 

 become isolated from their fellows, and amid slightly 

 differing surroundings they may undergo gradual changes 

 which ultimately transform them into a distinct type. 

 In this way one widely distributed species becomes 

 broken up into a number of new ones, each of which is 

 separated by some sort of barrier from the others. 



Animals of Oceanic Islands. — The laws of geo- 

 graphical distribution are achnirably illustrated by the 

 history of many isolated volcanic islands that most 

 certainly have never been united with a continent, and 

 accordingly have been colonized by individuals carried 

 by wind or water from some distant land mass. The 

 Galapagos Group, situated more than five hundred miles 

 west of Peru, is inhabited by animals, some of which 

 are identical with those of the mainland, while others 

 exhibit various degrees of modification. The land tor- 

 toises, lizards and snakes are all distinct species, and the 

 same is true of more than half of the insects and snails. 

 Where related forms are known these are invariably 

 South American. The same general facts appear in the 

 case of the Azores, though here the animals resemble 

 those of Europe nine hundred miles distant. The species 

 of the Bermudas are akin to those of America, while 



