CH. IV] ORIGIN OP GALAPAGOS. 209 



Galapagos with the mainland. A further argument in 

 the same direction is offered by the fact that the fauna of 

 the mountain peaks of the Galapagos at a height of 900 

 feet are like that of the mainland at 9000 feet. Given a 

 great depression this would be accounted for. That view 

 of the matter clears up in a satisfactory way the difficulty 

 of the tortoises; it would be inferred from this way of 

 looking at the problem that the huge Chelonians of the 

 Galapagos like those of Aldabra had been able to hold 

 their own by isolation and consequent freedom from 

 competition or persecution. Mr Wallace however is of 

 opinion that this assumption is unnecessary. "Con- 

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

 he remarks, " and the large number of allied forms which 

 have aquatic or subaquatic 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." The almost total disappearance of 

 mammals requires explanation on the hypothesis that the 

 Galapagos are a disjointed piece of South America. 



The study of the distribution of animals thus throws an 

 important light upon geological problems, and tends to 

 support the belief that animal life has come into being 

 upon large continental tracts and that it is liable to 

 variation when isolation is brought to bear. A careful 

 comparison of island fauna moreover enables some esti- 

 mate, chiefly of course comparative, to be made concerning 

 the age of the island in question. 



B. z. 14 



