262 HABITS AND INSTINCTS OF ANIMALS. CHAP. VIII. 



anxiety : when, however, the time of their departure 

 arrives, they become again quiet, and move off, generally 

 in the night, in a large body. 



(275.) Among land birds, some perform partial 

 migrations, moving their quarters from one part of 

 the same country to another, as their necessities may 

 prompt. Among these, the fieldfare is best known : 

 Dr. Jenner observes, " The occasional departure of 

 these birds, and some other of our winter inhabitants, 

 during a long-continued frost, must be very obvious. 

 The greater number disappear soon after its commence- 

 ment, if it sets in very severely: some few are always 

 left behind, arid are soon starved, if not fortunately re- 

 lieved by a thaw. Those that are driven to this neces- 

 sitous migration, probably, pursue a tract that quickly 

 leads them out of the reach of frost, or, at least, to 

 places which furnish them with food. The migration 

 of the soft-billed, or small insectivorous birds, is cer- 

 tainly astonishing ; and how their frame is enabled to 

 sustain those convulsions of the elements which they 

 must sometimes encounter, is altogether inexplicable. 

 Pennant remarks, that the golden- crested wren, dimi- 

 nutive as it is, in its migrations to the Shetland Isles, 

 in summer, accomplishes a flight of fifty miles, which, 

 he observes, must be entirely without interruption, 

 unless it should stop midway on Fair Island to rest.* 



(276.) A very large proportion of the marine FISHES 

 are more or less migratory, traversing the ocean at 

 particular seasons of the year, sometimes in immense 

 bodies, and gliding through the " vasty deep " with 

 astonishing velocity. It was long supposed that the 

 herring was of this number ; that it habitually lived in 

 the arctic seas, and came to our coasts in immense 

 armies. From a few partial facts favouring this sup- 

 position, Pennant framed such a lively and interesting 

 narrative, that the migrations of the herring form a 

 prominent subject in nearly all our popular publica- 

 tions. It is now, however, well ascertained, that this 



* Arctic Zool. vol. i. p. 29. 



