82 HIRUNDO RUSTICA. 



produced in various ways — Temporary migration, Permanent migration, 

 Partial migration, and Passive migration. They differ in their nature. 



Now, of the first, we may select that of the Swallow QHirundo rustled) 

 as the type. This bird is not English ; it is an African species which comes 

 to us for a purpose, urged by migratory impulse or fever, and intends to return 

 to its own country. 



Permanent migration is when a young Eagle or Kingfisher, after the 

 cessation of aropyrj on the part of its parents, finds itself driven out to get 

 its own living in a fresh country. This bird feels no migratory impulse. 

 Again, when a flock of Sky-Larks on the Continent is astonished by a fall of 

 snow, and show their alarm, they feel no migratory impulse ; fear and 

 hunger impel them. They cross perhaps to England ; at all events they fly 

 south before the pressure of the snow, which they are influenced by before it 

 comes. This is still temporary migration, but without migratory impulse. 

 They return ; but not in a flock. 



Partial migration I alluded to in Part. II. (pp. 82, 83), though I did not 

 give it a name. No doubt there is an immense deal of partial migration ; and 

 this may arise from migratory impulse or other causes. Of partial migration 

 the Goldfinch (^Fringilla carduelis) is an instance. 



Passive migration. — Of this the Skye seeds are an example; others 

 are mentioned in Haeckel, vol. i. p. 357. 



The above-quoted author proceeds thus :— " The means of transport are 

 partly active, partly passive ; that is to say, the organism eff^ects its migration 

 partly by free locomotion due to its own activity, and partly by the move- 

 ments of other natural bodies in which it has no active share.' ^ 



Again (p. 356) : — '' The fixed animals also (such as corals, tuberculous 

 worms, sea-squirts, lily-encrinites, sea-acorns, barnacles, and many other 

 lower animals which adhere to sea- weeds, stones, &c.) enjoy, at least at an 

 early period of life, free locomotion. They all migrate before they adhere to 

 any thing. . . . Many plants likewise enjoy it. Many lower aquatic plants, 



