383 



as for instance the Bath district, are few in number, and the life 

 history of most of them is tolerably well understood. The birds 

 are more numerous, and the particular species we may expect to 

 meet with more uncertain. From their powers of flight, and 

 from various circumstances, some of regular occurrence, others 

 accidental, birds often wander far and wide from the place of their 

 nativity. Unlike mammals, more fixed to the spot, they can in a 

 very short time transport themselves elsewhere in the case of food 

 failing, or undur any other influences which affect their welfare. 

 Hence it occasionally happens that while some species desert a 

 particular locality, others new to it introduce themselves in 

 their stead. Nor can we always argue that because a country 

 or district seems well suited to a particular species it is likely to 

 occur there. For no accident may have brought it over the 

 boundary that separates that country or district from an adjoining 

 one, in whatever plenty it may be found in this latter. The rule 

 seems to be that having once got a footing in any locality, if it 

 find everything there requisite for its support and safety, unless it 

 be a migi-atory species, there it will remain ; and even if it do 

 migrate to any other place at certain seasons, unless hindered in 

 its flight by opposing circumstances, it will return to the locahty it 

 had left. It has been noted as a remarkable circumstance in the 

 geographical distribution of birds that, notwithstanding the large 

 rivers and other waters in America, " a continent with more rivers 

 and more fish than any other," there should be so few kingfishers ; 

 whereas in the Australian regions, where rivers are smaller and 

 less numerous, kingfishers abound. We even find there nearly 

 half the species of the whole world.* We may easily suppose, 

 however, Australia to have been the centre in which, from some 

 particular cause, many species originated, and that nothing has led 

 to the dispersal of such species over other parts of the globe. 



But to take a case nearer home ; we know that the nightingale, 

 which is a migrant, returns to the same parts of England every 

 spring, leaving other parts year after year unvisited, though in 

 many of the counties which it seems to avoid there is nothing we can 

 fix upon as prejudicial to its welfare if it chose to resort to them. 

 * See " Nature," vol. iii., p. 467. 



