XIII EVOLUTION AND DISTRIBUTION OF ANIMALS 281 



Distribution of Gigantic Land-tortoises. 



The last case of anomalous distribution — that of the 

 giant land-tortoises of the Mascarene and Galapagos 

 islands — offers perhaps less real difficulty than any of the 

 preceding, on account of the existing wide range and the 

 extreme antiquity of the genus Testudo to which they all 

 belong. This genus occurs in the Miocene deposits of 

 Europe and India, and in the Eocene of North America ; 

 and living species are abundant in Africa and Asia, and 

 are also found in South Europe and North America. It 

 has evidently, therefore, been a dominant group during a 

 large portion of the tertiary epoch, and it still maintains 

 a vigorous existence. There does not seem to be any 

 evidence that these giant species of the two hemispheres 

 are more closely allied to each other than the smaller 

 forms of remote regions ; for though the Galapagos 

 tortoises and the extinct species of the Mascarene islands 

 both belong to a flat-headed type, they may have differed 

 in important external characters. Their gigantic size is 

 evidently due to their seclusion for countless generations 

 in islands where they were entirely free from the attacks 

 of enemies, and where they could procure abundance of 

 food; both natural and sexual selection giving the ad- 

 vantage to the larger and stronger individuals. The only 

 difficulty is how they reached the Galapagos. But as we 

 may go back to the middle of the tertiary epoch for this 

 event, it is not an improbable supposition that the coast 

 of South America then extended considerably westward, 

 while the islands themselves may have been more 

 extensive, thus reducing the dividing strait to a width 

 across which either the adult animals or their eggs might 

 be floated by currents or surface-drifts. Their entrance to 

 the Mascarene islands from Africa might have been effected 

 in a similar manner. This is the solution suggested by 

 Dr. Gtinther himself,^ and it is one which perfectly har- 

 monises all the known facts. 



^ Nature, vol. xii. p. 297. 



