GEOGRAPHIC DISTRIBUTION OF ANIMALS: OCEANIC ISLANDS 289 



brought down in one day two hundred tortoises to the beach." The size 

 and strength of the creatures is indicated by the fact that people can ride 

 on their backs, a pastime indulged in by Darwin, who wrote, "I frequently 

 got on their backs, and then giving a few raps on the hinder part of their 

 shells, they would rise up and walk away; — but I found it very difficult to 

 keep my balance." 



Two other distinctive reptiles of the Galapagos Islands are the land, 

 and marine iguanas. These big, lizardlike creatures are from 3 to 4 feet 

 long. They contribute much to the feeling experienced by visitors of having 

 stepped back into Mesozoic times. Formerly both varieties were extremely 

 abundant. The marine iguanas are still found in large numbers, but the 

 terrestrial iguanas are now nearly extinct. Yet of the latter Darwin wrote, "I 

 cannot give a more forcible proof of their numbers, than by stating that 

 when we were left at James Island, we could not for some time find a spot 

 free from their burrows on which to pitch our single tent." That was in 

 1835. 



The land iguanas are rather brightly colored, brownish red above, yellow 

 underneath. The marine iguanas, on the other hand, are black. Both species 

 are vegetarians, the marine form living on green algae, the land form on a 

 variety of plant material, such as cactus, and the leaves of acacia trees. 



The marine iguana is particularly remarkable as being the only known 

 lizard to lead an aquatic existence. As adaptations for this mode of life it 

 has partially webbed feet and a laterally flattened tail. It swims by serpen- 

 tine movements of the body and tail, after the manner of the most accom- 

 plished swimmers in all classes of vertebrates, except birds. It seems 

 reasonable to infer that when ancestral iguanas reached the islands they 

 increased greatly in numbers until eventually the available food supply on 

 land was inadequate to support further expansion. Under such conditions 

 the pressure on the food supply would have been relieved if some of the 

 iguanas proved capable of taking advantage of the algae abundant in 

 neighboring shore waters. We may well imagine that the splitting of the 

 iguana stock into the two forms found today occurred under such an 

 impetus. 



The roll of terrestrial reptiles on these islands is completed by mention 

 of the fact that there is one genus each of snake, small lizard, and gecko. 



Mammals 



The disharmonic nature of the fauna is still further attested by the mam- 

 mals, or rather by the lack of them. There is one genus of bats and one of 



