THE MIGRATION OF INLAND BIRDS. 123 



southward they proceed, until they reach a home in lands 

 blessed with perpetual summer. 



I have now traced these migratory species from south 

 to north, and back to their southern habitat, and endeav- 

 ored to point out the several operating causes of the 

 movement as I did so. I have already suggested the 

 possibility of migration being an inherited habit not now 

 necessary. Now, be this true or not, it is evident that 

 the habit is not so fixed that ordinary changes in sur- 

 rounding conditions do not greatly influence it. This, I 

 think, is shown by the irregularity of the movement that 

 really occurs, and the tendency on the part of many 

 species to modify the habit by occasionally halting much 

 to the south of their usual breeding-grounds, and by re- 

 maining later and later in autumn; and, again, by the 

 fact that many birds are now only partially migratory, 

 and that others occasionally migrate simply in search of 

 food, irrespective of seasons, thus exhibiting, as it were, 

 traces of a habit they have long lost. 



In the migration of a bird, then, I see simply a tem- 

 porary sojourn in a distant locality for the purpose of 

 rearing its offspring in safety ; the cause being implied 

 by the term "safety," that is, freedom from enemies and 

 an abundance of food. 



