TRISTAN DA CUXHA. 133 



enormous. At least one-fourth of the surface of the island and 

 small outlyers, for these also are rookeries, must be covered 

 by them ; taking thus a space a quarter of a mile square, and 

 allowing two only to a square yard, there would be nearly 

 400,000 penguins. 



The rookery has evidently once been larger than at present, 

 since a good part of the tall grass, now not occupied by birds, 

 had old deserted nests amongst it. Probably the number of 

 birds varies considerably each season. 



One of the most remarkable facts about the penguins is that 

 they are migratory ; they leave Inaccessible Island, as the 

 Germans told us, in the middle of April after moulting, and 

 return, the males in the last week of July, the females about 

 August 12th ; and I do not think it possible that the Germans 

 could have been mistaken. Whither can they go, and by what 

 means can they find their way back ? The question with 

 regard to birds that fly is difficult enough, but it may always be 

 supposed that they steer their course by landmarks seen at 

 great distances from great heights, or that they follow definite 

 lines of land. In the present case the birds can have abso- 

 lutely no landmarks, since from sea level Tristan da Cunha is 

 not visible from any great distance; the birds cannot move 

 through the water with anything approaching the velocity of 

 birds of flight ; they have however, the advantage of a constant 

 presence of food. The question of the aquatic migration of 

 penguins and seals seems a special one, and presents quite 

 different difficulties to that of the migration of birds of flight. 

 The penguins certainly do not go to the Cape of Good Hope nor 

 St. Helena, and they cannot live at sea altogether. 



The migration of the turtles at Ascension Islands, seems to 

 be possibly a parallel case. The young turtles on leaving the 

 egg go down to the sea and disappear, returning only when full 

 crown to breed ; this is the account given bv residents. If 

 they do really leave the neighbourhood of the island, there seems 

 no possible means by which they can find their way back. 



There is little fresh water on Nightingale Island. I saw one 

 pond in the rookery, but the water was undrinkable. In a 

 cave, however, where we landed, there was a scanty trickling 



