AI'PKXDIX. 359 



tioiis." " The unquietiKk'," says anotlier aiitlior, " wliicli they 

 manifest might in case of need serve as an almanack." " The 

 shepherds must then exert all their vigilance to prevent them 

 escaping," " for it is a known truth that they would go to the 

 very place wdiere they had heen born." ]\Iany cases have 

 occurred of three or four sheep having started and performed 

 the journey by themselves, though generally tliese Avanderers 

 are destroyed by the wolves. It is very doubtful whether 

 these migratory slieep are aborigines of the country ; and it 

 is certain that within a comparatively recent period their 

 migrations have been widely extended: this being the case, I 

 think there can hardly be a doubt that this " natural instinct," 

 as one author calls it, to migrate at one particular season in 

 one direction has been acquii'ed during domestication, based 

 no doubt on that passionate desire to return to their birth- 

 place which, as we have seen, is common to many breeds of 

 sheep. The whole case seems to me strictly parallel to the 

 mio-rations of wild animals. 



Let us now consider how the more remarkable migrations 

 could possibly have originated. Take the case of a bird being 

 driven each year, by cold or want of food, slowly to travel 

 northward, as is the case with some birds ; and in time we 

 may well beheve that this compulsory traAclling would 

 become an instinctive passion, as with the sheep in Spain. 

 Xow during the long course of ages, let valleys become con- 

 verted into estuaries, and then into wider and wider arms of 

 the sea ; and still I can well believe that the impulse which 

 leads the pinioned goose to scramble northward would lead 

 our bird over the trackless waters ; and that, by the aid of 

 the unknown power by which many animals (and savage 

 men) can retain a true coiu'se, it would safely cross the 

 sea now covering the submerged path of its ancient land 

 journey.* 



* I do not suppose that the line of migration of birds always marks the 

 line of formerly continuous land. It is possible that a bird accidentally 

 blown to a distant land or island, after staying some time and breeding there, 

 might be induced by its innate instinct to fly away, and again to return there 

 in the breeding season. But I know of no facts to countenance the idea ; 

 and I have been much struck in the case of oceanic islands, lying at no ex- 

 cessive distance from the mainland, but which from reasons to be given in a 

 future chapter I do not believe have ever been joined to the mainland, with 

 the fact that they seem most rarely to have any migratory birds. Mr. E. V. 

 Harcom't, avIio has written on the birds of Madeira, informs me that there 

 are none in that island ; so, I am infonned by Mr. Carew Hunt, it is in the 



