262 Transactions. — Zoology. 



The reason is obvious. As these islands have never formed 

 part of a continent they never could have been either the 

 breeding-places or the feeding-ground of migratory birds. No 

 birds visit them instinctively, and if any bird happens to 

 wander to one of them it has no hereditary instinct direct- 

 ing it how to return. This is why oceanic islands have no- 

 migratory birds. 



The Polynesian whimbrel (Numcnius tahitiensis) must 

 breed in some of the Polynesian islands, for it is not known 

 elsewhere, except as a straggler in British North-west 

 America. It is closely related to the American whimbrel, 

 and no doubt it is descended from some stragglers of that 

 species which, long ago, found their way to the Hawaiian 

 islands. 



The only possible explanation of oversea migration seems 

 to be that the birds are following old land-lines. The shore- 

 birds follow the old shore-line ; the land-birds follow the old 

 land. Migration must have commenced when the two lands 

 were contiguous, or nearly so, so that in no part of the course 

 was an island so far off as to be invisible from those next to 

 it. Gradually the land sank, but the force of habit kept up 

 the migration. During the life of each bird the changes 

 would be too small to be perceptible, and it would only be 

 after many generations that the birds would find themselves 

 flying over a trackless ocean. That migration is really an 

 instinct we know from the fact that in the Northern Hemi- 

 sphere the birds start on their flight to the south before there 

 is any real necessity for it. The young cuckoos, which, of 

 course, have never seen their parents, do not leave until 

 two months after the old ones have gone, and, as they have 

 none to lead them, their southern flight must be due to 

 inherited memory. 



But how do they find their way ? What is the sense 

 which directs them during their flight ? This is a question 

 which it is at present impossible to answer. Nevertheless, it 

 is a matter of fact that they do find their way. The faculty, 

 whatever it may be, is not an uncommon one in the animal 

 kingdom. We see it exercised to a small extent by bees and 

 ants and many other insects. Sea-snakes and turtles return to 

 the same place to breed, although during their absence from 

 the land they must have swum many miles in many different 

 directions. Penguins, petrels, and other sea-birds have the 

 same faculty ; as also have seals, several of which perform 

 long oceanic migrations. It is also well known that some 

 domesticated animals can return to their homes after having 

 been taken long distances. And savages, after following 

 their quarry for several hours, or even for days, can always 

 find their way home again through dense forest. 



