GEOGRAPHIC DISTRIBUTION OF ANIMALS: OCEANIC ISLANDS 291 



given to which all students of the islands will agree. The depth of the ocean 

 between the islands and South America is greater than the depth of a sec- 

 tion of the ocean extending northeast to Cocos Island, in the direction of 

 Central America (Fig. 13.2). The view that the islands were once con- 

 nected to Central America has received favorable discussion by Beebe 

 (1924). 



To many other students of the subject the amount of vertical movement 

 of the crust necessary to form a dry-land connection to America seems 

 unlikely, during that portion of geologic time concerned in the distribution 

 of immediate ancestors of the plants and animals inhabiting Galapagos. 

 Also, as we have seen, the flora and fauna are notably disharmonic, a fact 

 favoring the view that immigration across the ocean was the means of 

 population. The affinity of the Galapagos fauna to that of Central America 

 is attested by many investigations. C. T. Parsons (personal communica- 

 tion) is of the opinion that ancestral forms were carried by the Panamanian 

 current, which swings south to include the archipelago every few years. 

 "This current washes up many floating plants and brings much rain which 

 would enable many plants and animals, thrown up on the usually barren 

 shore, to gain a foothold." 



Opponents of the view that plants and animals reached the archipelago 

 by transportation across water have raised particular objection to the sug- 

 gestion that land iguanas and land tortoises could have reached the islands 

 in this way. Yet lizards, the group to which iguanas belong, have colonized 

 most of Polynesia, including very isolated islands. Apparently they can be 

 carried long distances on floating vegetation, or even perhaps floating in the 

 water themselves. Much remains to be learned about the means of dispersal 

 possible to any particular animal. Tortoises have been observed to float 

 and survive for long periods in sea water. Simpson ( 1943 ) has pointed out 

 that the great land tortoises probably reached both South America and 

 the Galapagos Islands in this manner, since they first appeared in South 

 America in Miocene times, when that continent was not connected by land 

 with North America. Some other islands (e.g., the Mascarene Islands in 

 the Indian Ocean) also have giant land tortoises despite the fact that no 

 evidence exists of former land connection to a continent. 



While the aflfinities of the Galapagos fauna are almost exclusively with 

 the American fauna, one Polynesian form is included. This is a land 

 mollusc, unrelated to those in America and apparently derived from islands 

 at least 3000 miles to the west. It seems wiser to state that we do not know 

 the means by which this creature reached the islands than to postulate a 

 far-flung land bridge for its exclusive use. 



