326 POPULAR SCIENCE MONTHLY. 



The theory that is, perhaps, most naturally suggested, and the one 

 that finds widest acceptance as explaining the facts is that migration 

 began in a search for food. That is, the food supply becoming short in 

 the vicinity of the home (a bird's home is thus assumed to be the 

 place where it rears its young, and may therefore be quite different 

 from the locality where it spends the remainder of the time) they 

 wandered away in search of food, returning again and again to the 

 home vicinity. These journeys were extended farther and farther, 

 the birds returning each nesting season, undoubtedly oftener at first, 

 to or near the locality where they were born. This process went on 

 until their wandering became a fixed habit, and finally in the count- 

 less generations of birds that have come and gone, this habit has 

 been crystallized into what we now call, for want of a better term, the 

 instinct of migration. 



This idea has been amplified and extended by Alfred Eussell 

 Wallace ('Nature,' X., p. 459) . He supposed that 'survival of the fittest' 

 has probably exerted a powerful influence in weeding out certain 

 individuals. He supposed further that breeding can only be safely 

 accomplished as a rule in a given area, and that during a greater part 

 of the rest of the year sufficient food cannot be obtained in that area. 

 "It will follow that those birds which do not leave the breeding area at 

 the proper season will suffer, and ultimately become extinct; which 

 will also be the fate of those which do not leave the feeding area at the 

 proper time." His further argument is ingenious, and, it must be 

 added, extremely plausible. He says: "Now, if we suppose that the 

 two areas were (for some remote ancestor of the existing species) 

 coincident, but by geological and climatic changes gradually diverted 

 from each other, we can easily understand how the habit of incipient 

 and partial migration at the proper seasons would at last become 

 hereditary, and so fixed as to be what we term an instinct." 



It will probably be found, however, if anything like a satisfactory 

 explanation can be arrived at, that this habit or instinct has arisen in 

 more than one way, but we may appropriately turn from a consider- 

 ation of theories to a review of certain observed facts of migration. 



It is now abundantly established that migration is mostly carried 

 on at night, and further mainly during clear nights. Only a com- 

 paratively few species, such as ducks, cranes, certain large hawks, swal- 

 lows, swifts, and nighthawks, migrate during the daytime, and these 

 it will be observed, are either rapacious birds or mainly those that 

 enjoy such power of rapid flight as to be relatively safe from capture. 

 All the vast horde of warblers, sparrows, finches, flycatchers, thrushes 

 and woodpeckers, as well as many waders and swimmers, migrate at 

 night. On clear, still nights during the migrations birds may often 

 be heard calling to each other high over head, and, as will be described 



