22 HISTORY OF THE EUROPEAN FAUNA. 



of land-reptiles to islands have actually been observed. 

 But the fact of the occurrence of such instances by no 

 means proves that reptiles thus conveyed are able to 

 establish themselves permanently in their new home. 

 Sir Charles Lyell records in his Principles of Geology 

 that a large boa-constrictor was once seen floating to 

 the island of St. Vincent, twisted round the trunk of 

 a tree. It appeared so little injured by its long 

 voyage from South America, that it captured some 

 sheep before it was killed. 



Mammals might be accidentally conveyed to islands 

 on such rafts as have been described by Sir Charles 

 Lyell, and there are instances on record of their 

 having crossed short distances of sea by swimming. 

 Elephants and also deer and pigs are good swimmers, 

 the former having been known to swim for six hours 

 at a stretch. "But," remarks Mr. Lydekker (p. 13), 

 "it may be assumed that about twenty miles is the 

 utmost limit which mammals are likely to cross by 

 swimming, even when favoured by currents. Such 

 passages as these must, however, be of very rare 

 occurrence, for a terrestrial mammal is not likely 

 to take it into its head to swim straight out to sea 

 in an unknown direction. Moreover, supposing a 

 mammal, near to a particular island, to have arrived 

 there by swimming, unless it happen to be a pregnant 

 female, or unless another individual of the same 

 species but of the opposite sex should arrive soon 

 after (a most unlikely event), it would in due course 

 die without being able to propagate its kind." 



