Vol. II, Pt. I] VAN DEXBURGH— GALAPAGOS TORTOISES 



367 



ble, James, Jervis, Duncan, Narborough, and Albemarle 

 islands. The last named island supports several races of tor- 

 toises. The tortoises of Barrington are practically unknown. 

 Each of the other nine islands had its own peculiar race of 

 tortoise, and on none of these nine islands has evidence of more 

 than one race been found. This lends particular interest to 

 the fact that several races of tortoises occur on Albemarle 

 Island. 



Although these tortoises can live for at least several days 

 floating on the surface of the ocean, they are absolutely help- 

 less in the w'ater. They are unable to swim, and can only 

 float and drift at the mercy of the winds and currents. \Mien 

 they drift on island shores, they usually are so battered and 

 injured on the rocks that they only live a few days there- 

 after. The fact that each island, except Albemarle, has one 

 and only one race of tortoise, is evidence that interchange of 

 tortoises betw^een the islands has not occurred, for such inter- 

 change would result either in preventing differentiation or in 

 the presence of more than one race on an island. 



If the transportation of tortoises from one island to another 

 does not occur, there is little reason to believe that tortoises, 

 at some time in the past, have drifted over the vastly greater 

 distance from some continent, and have reached each of the 

 eleven islands on which they have been found. Nor do we 

 know whence they could have come. The evidence offered by 

 these tortoises, therefore, seems to be against the view that 

 these are oceanic islands, which have been independently 

 thrust above the surface of the water, and have received such 

 animals as have drifted to them. We must rather adopt the 

 view^ that the islands are but the remains of a larger land- 

 mass which formerly occupied this region, and was inhabited 

 by tortoises, probably of but one race ; that the gradual partial 

 submersion of this land separated its higher portions into vari- 

 ous islands; and that the resulting isolation of the tortoises 

 upon these islands has permitted their differentiation into dis- 

 tinct races or species. 



If isolation plays so prominent a part in the differentia- 

 tion of species in the Galapagos Archipelago, and each island 

 has its one distinct race, what is the explanation of the fact 

 that on Albemarle Island five distinct races of tortoises occur? 



September 30, 1914. 



