MIGRATION IN ITS BEARING ON LAMARCKISM 1 07 



birds, descended from lost wanderers who, finding these spots 

 unoccupied, have been able to establish themselves. 



The list of North American birds which are occasionally found 

 in Europe is a long one, and stray specimens of the gray plover, 

 whose summer home is the shore of the Arctic Ocean, have been 

 found at the Cape of Good Hope, in Ceylon, in Australia, in New 

 Zealand, and in Tasmania. Most of the wanderers are shore birds 

 which make long migrations and, being much exposed to storms, 

 are often driven far out of their path, but this is not always the 

 case, for the great albatross follows ships across the whole breadth 

 of the South Pacific, or nearly half the circumference of the 

 earth. Many birds seem to make their whole journey by a single 

 flight, for some which are common in the West Indies and in 

 Nova Scotia are almost unknown within the limits of the United 

 States, making the whole journey past our borders by water and 

 probably by a single flight. The blue-throat, which breeds in the 

 northern part- of Scandinavia, is so seldom found in Europe south 

 of the Baltic that there seems to be good evidence that it makes 

 its whole journey to its winter quarters, which are in the region 

 of the upper Nile, by a single flight. 



There is no reason to suppose that all migratory birds inherit 

 the habit from a common source, or that its purpose is always 

 the same; and many birds of prey seem to have acquired the 

 habit of ranging far in winter in search of food, and of following 

 their prey into warmer regions, to return to their birthplace in 

 seasons of reproduction. In these cases the birthplace may have 

 been the original home, before the migratory habit was acquired, 

 and the scarcity of food the reason why it was acquired ; and the 

 influence of scarcity in causing migration is well shown by the 

 occasional migrations of certain prolific animals which do not 

 ordinarily leave their birthplaces, although, when these become 

 overstocked, migrations take place, just as colonies are sent out 

 by the people of thickly settled countries to find new room for 

 growth in foreign lands. From time to time, at irregular inter- 

 vals, great armies of the smaller and more prolific rodents, which 

 usually spend their lives where they were born, are met with on 

 the march from homes where overproduction has exhausted the 

 food; and several of the older American naturalists have described 



