194 THE POPULAR SCIENCE MONTHLY. 



tion, reared their broods, clearly, if all things needed for their comfort 

 were to be obtained, it cannot be supposed that these same birds would 

 unnecessarily retrace their long flight to the distant South. This sug- 

 gests that if we are correct in assuming that birds first ajjpeared in a 

 tropical climate, and from such climate migration started, it is proba- 

 ble that by gradually prolonging their northern visits and accustom- 

 in 2: themselves to northern insect and veo-etable life, these reaions 

 became populated by their resident species. It is evident that the 

 present migratory species are simply compelled to return, and three 

 compelling causes are demonstrable. Primarily, the sudden increase 

 of cold at the close of the brief northern summer, which starts south- 

 ward those farthest at the north. This accession of intense cold ne- 

 cessarily decreases the amount of food, and the birds are now forced to 

 find it elsewhere. Farther and farther south they come, just in ad- 

 vance of the cold, and slower and slower they proceed, as they enter 

 our more temperate latitude, and here, resting as it were, they linger 

 until a keen frost kills their insect-food, and, scattering the leaves, 

 robs them of their main shelter from their enemies, happily fewer now 

 than formerly ; and now still southward they proceed, until they reach 

 a home in lands blessed with perpetual summer. 



We have now traced these migratory species from south to north, 

 and back to their southern habitat, and endeavored to point out the 

 several operating causes of the movement as we did so. We 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 a one that ordinary changes in surrounding 

 conditions do not greatly influence it. This, we 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 halt- 

 ing 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 others by occasionally 

 migrating simply in search of food, thus exhibiting, as it were, traces 

 of a habit they have long lost, as to its full meaning and accom- 

 plishment. 



In the migration of a bird, then, we see simply a temporary sojourn 

 in a distant locality for the purpose of rearing its oflspring in safety ; 

 the cause being implied by the term " safety," i. e., freedom from ene- 

 mies, and an abundance of food. 



