GIANT TORTOISES AND THEIR DISTRIBUTION 313 



group were known. This being so, it remains to consider 

 how the transit was effected. 



Confining attention in the first instance to the Galapagos 

 group, it has to be mentioned that both Darwin and Wallace 

 are in accord in referring these islands to the " oceanic " type ; 

 that is to say, in regarding them as having been thrust up 

 from the bed of the ocean, and therefore isolated throughout 

 the whole period of their existence alike from one another 

 and from the nearest mainland, so that the ancestors of their 

 present scanty fauna arrived by crossing the sea. 



Now the Galapagos group, which occupies an area of about 

 300 by 200 miles, comprises five large and twelve small 

 islands composed of volcanic rocks, lying about 600 miles 

 westward of the coast of Equador. With the exception of 

 two or three species of rats and mice, their terrestrial vertebrate 

 fauna consists solely of giant tortoises, with certain kinds of 

 lizards and snakes of a South American type but in some cases 

 representing peculiar genera. 



To account for the presence of the tortoises, Dr. Wallace, 

 on page 268 of Island Life, makes the following suggestion : 

 " Considering the well-known tenacity of life of these animals, 

 and the large number of allied forms which have aquatic or 

 sub-aquatic habits, it is not a very extravagant supposition 

 that some ancestral form, carried out to sea by a flood, was 

 once or twice safely drifted as far as the Galapagos, and thus 

 originated the races which now inhabit them." 



This statement contains, I venture to think, one crucial 

 fallacy, namely, that the ancestral Galapagos species was not 

 a giant tortoise at all, 'whilst there appears to be a suggestion 

 that it was not wholly terrestrial in habits. The fact that giant 

 tortoises were distributed in Tertiary times over the greater 

 portion of the warmer regions of the globe, coupled with the 

 affinity— especially as regards colour — of the Galapagos tortoises, 

 to some of those of the islands of the Indian Ocean, indicates, 

 however, without reasonable doubt that the former were 

 " giants " when they reached their present habitat, F'urther, 

 the peculiarity in the structure of the cervical vertebrae of 

 one of the species mentioned above — a peculiarity, it may be 

 repeated, shared by the Aldabra forms — is highly suggestive 

 that there were at least two ancestral species which reached 

 the Galapagos from the South American mainland. 



