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



369 



this differentiation varies in degree on the various islands — 

 being greatest usually on the more out-lying islands — it may 

 be regarded as an index to the order of separation of the 

 various islands, and evidence of their gradual depression. Only 

 on Albemarle Island do the reptiles suggest that there has been 

 land elevation, and even here the recent period of rising must 

 have been preceded by a long period of depression, during 

 which the present Albemarle Island was represented, prob- 

 ably, by five separate islands corresponding to its five great 

 volcanoes. 



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