432 ANIMALS INTRODUCED UNAWARES 



although the presence of man, with his stores of food and 

 litter of garbage, is the great determining factor in the dis- 

 tribution of the common species. Wherever the carcase is, 

 there will the Rats be gathered together, and so they occur 

 in great hordes far from man's habitations, infesting the cliffs 

 of Caithness and other parts, and, as Mr O. H. Wild tells me, 

 many isolated and uninhabited islands off our coasts, as 

 Craigleith off North Berwick, the Gaelic-named "Home 

 of the Bird" off Isle of Muck, Puffin Island near Anglesey, 

 and others; for Rats are excellent swimmers and have made 

 their way to many a solitary isle. In these cases the Rats 

 depend on the eggs, young and even adults of the birds 

 which frequent the coast or islands, for they live in the bur- 

 rows of the Puffins, enjoying rich fare during the breeding 

 season and managing to scrape along during the winter on 

 the molluscan and crustacean shell-fish and the refuse of the 

 seashore, until spring brings again birds and plenty. 



Even during the temporary breeding season, however, 

 the Rats must have an appreciable influence in restricting the 

 bird-colonies, and it is possible that their introduction may 

 have had more to do with banishing and exterminating 

 ground-nesting birds from closed areas like islands than 

 one generally attributes to it. Even the smaller ground 

 mammals may suffer from the Rat's voracity, for it is one of 

 the worst foes of the Rabbit, hunting down the young in their 

 burrows with the savage pertinacity of a ferret. 



There is a type of ship-voyager, very different from the 

 Rats, and perhaps of not much account in the make-up of a 

 fauna, which clings to the exterior of a vessel and with it is 

 taken from sea to sea. I think of those borers in the timbers 

 of wooden ships the Teredo or so-called Ship- Worm a 

 bivalve mollusc, and of the creatures which form a crust or 

 a coat upon the external surface of hulls whether they be of 

 wood, copper or iron stony Barnacles and stinging Hydro- 

 zoa. It is almost impossible now to disentangle from the 

 complicated net of Nature's facts the threads of influence 

 upon the spread of these marine creatures created by the 

 trafficking of ships ; but many of them have a suspiciously 

 wide distribution the Common Acorn Barnacle of ships 

 (Ba/anus tintinnabulum\ for example, has been found all 



