508 THE CHANGING WORLD 



Evidence from Distribution 



The peculiar way in which species of animals and plants are dis- 

 tributed in oceans and upon land finds no sensible explanation 

 unless it is assumed that evolution has occurred, when it plainly 

 becomes a matter of untangling historical events, such as past geo- 

 logical changes and the migrations of plants and animals, and finding 

 out their proper sequence. From their original home the members 

 of each species scatter, due to overcrowding, the search for food, 

 and various other reasons, until they encounter barriers that limit 

 their advance. The species may settle in a new habitat, or undergo 

 transformations and adaptations that make further exploration of 

 the world possible. A species may also perish in the attempt to 

 live in a changing environment to which it cannot adapt itself. 

 Fossil records are filled with examples of this sort. Therefore, the 

 key to the present distribution of organisms lies in a knowledge of the 

 vicissitudes experienced in the past. 



It was the unusual distribution of life on the Galapagos Islands 

 that started Charles Darwin in his yeasty thinking about evolution, 

 and, as everyone knows, he started others to thinking. These vol- 

 canic islands lie 500 miles off the coast of Ecuador, and were visited by 

 Darwin during his famous voyage around the world on the Beagle. 

 There he found an assemblage of peculiar animals, all unmistakably 

 patterned after South American forms, but yet modified somewhat 

 from the continental types. It is evident that originally there must 

 have been land connection between South America and what is 

 now this archipelago of volcanic islands, making a bridge over which 

 continental animals could migrate. With the gradual subsidence of 

 the oceanic floor, the tops of the volcanoes were left as isolated 

 islands, and the islanders found themselves cut olT from their rela- 

 tives on the mainland. Survival on these isolated islands called for 

 nice adaptation, different in each different habitat. 



South America and Africa have in general the same climate and 

 would be suitable habitats for the same organisms. Nevertheless, 

 the faunas and floras of these regions are quite different. In Africa 

 are found the rhinoceros, lion, wart hog, zebra, baboon, giraffe, 

 gorilla, okapi, and aardvark. None of these animals occur in South 

 America, which in turn is the home of the armadillo, sloth, vampire- 

 bat, llama, peccary, tapir, agouti, and marmoset, not one of which 

 is found in Africa. Such diverse distribution indicates that these 



