6958 Birds. 



species. These were : — That all species require a certain fixed 

 standard amount of the great physical agents for their due develop- 

 ment ; that this standard may be altered within certain limits with- 

 out destruction to the species, though at the expense of its well-being 

 [range of existence) ; and lastly, that there are certain fixed limits to 

 this range outside of which the species must absolutely perish. Now, 

 remembering that the standard of existence is not necessarily uniform 

 in different species, nor even at different ages of the same species 

 (the standard of existence in the adult bird having a more extensive 

 range than in the young), and bearing in mind that the food of the 

 two differs greatly in quantity, periodic migrations that is, those 

 strongly-marked passages, at fixed periods of the year, of species from 

 one area to another, are easily explained, excluding at present those 

 migrations in which the passage performed is merely a shifting from 

 one district to another similar one, necessitated by the failure of 

 food. 



Taking such a view of the case, migration resolves itself into this : 

 a species (the whitefronted goose, Anser albifrons, for instance) rears 

 its young in the North during the summer season of that region, when 

 food of the kind suitable for those young is easily procurable ; after 

 the young are fully developed winter sets in, and either destroys 

 that food or renders it unattainable or nearly so. The species, flying 

 the winter, travels south ; finding in its course conditions pretty- 

 similar to those which prevailed in its summer abode in more northern 

 latitudes : when at length it has reached a district (suppose Great 

 Britain) in which these conditions, or at least conditions compatible 

 with its adult existence are permanent in the winter, it there abides 

 until the increasing heat of the spring renders its adopted home un- 

 suited to it, or, at least, to its future progeny. Again it takes up its 

 journey, travels north, flying from the summer heat; such a species 

 arriving here in the winter the Briton calls a winter migrant. A second 

 species (the house swallow, Hirundo rustica) rears its young in 

 Britain ; this duty over, on the appearance of the British winter, 

 it seeks in the milder latitudes of the South its winter quarters, 

 returning again to the North when these prove too hot to hold it; and 

 such a species the Briton calls a summer migrant. 



This explanation is not contradicted by the occasional breeding and 

 residence, through the entire year, of individuals in districts interme- 

 diate between the actual northern and southern hiemal and sestival 

 residences of the species, because it must be remembered that the 

 limits of the standard of existence of a species are sometimes very 



