282 .\\\\Ls \i:\v yoh'K \c.\ni:\iy or sriiJXCES 



or Masfarenhas, situated to tlie east of Madagascar and including Reunion, 

 Mauritius and Rodriguez and lastly tlie Amirantes and the Seychelles, wliich 

 are the most northern of tlie whole assemhinge and only about four degrees 

 south of the eciuator." ''* 



Each of tliese groups of islands, except the Mascarenes, stands upon 

 a shallow platform, and is surrounded by abyssal ocean, upwards of 5000 

 feet between the Comoros and Africa, elsewhere upwards of ten thousand 

 feet. The three Mascarene islands rise separately from abyssal depths. 

 Madagascar is about 180 miles from the African coast; the other islands 

 are 400 to 600 miles from Madagascar; the present normal set of current 

 is unfavorable to transportation from Madagascar. 



It is very frequently asserted that a bank of shallow soundings con- 

 nects India with Madagascar through the Amirante Seychelles group, 

 and that this indicates a former continental bridge of which these islands 

 are remnants. The facts are as above stated ; the so-called bank is vei7 

 little above the general level of the floor of the Indian Ocean and is not 

 differentiated from it in any features of relief that would suggest its 

 former continental character. 



The transportation of natural rafts five hundred miles against the 

 normal set of current — or five times that distance if from the East 

 Indies — is the most improbable element in this explanation. There is 

 no valid reason to suppose that the general direction of winds and cur- 

 rents differed materially in the later Tertiary from the present day 

 conditions. I do not think it necessary to assume with Dr. Lydekker 

 that the tortoises were of gigantic size when they reached the islands or 

 to ignore, as he does, the elements of parallelism in considering their 

 affinities to continental species. Xor does it appear that the difficulties 

 which he admits in accounting, on the hypothesis of former continental 

 union, for the absence of the rest of the fauna, should be "set aside for 

 future consideration." They add so greatly to the improbability of the 

 hypothesis, that in conjunction \vith the physiographic difficulties it 

 appears wholly out of range of reasonable probability. On the other 

 hand, an investigation of the very variable direction of the winds and 

 currents in the Indian Ocean would probably yield data to reduce the 

 improbabilities in the hypothesis of over-sea transportation as above 

 stated. The third possible hypothesis is that the present distribution is 

 due in part to human agency, not necessarily limited to the historic 

 period. If this factor may account for a species of Cams in Australia 

 distinct from the living species of Arctogaea, it may perhaps help to 

 account for peculiar species of tortoises as well. 



100 Science Progress. October, inin. p. 303. 



