122 RAMBLES ABOUT HOME. 



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varied from those of both other species of the same family 

 and from species of other families. For instance, to 

 avoid a common enemy, a number of species might have 

 gradually learned to migrate at night ; while others, al- 

 though forced to migrate, had not this same enemy to 

 contend with, and so traveled only by day. In this way 

 the habit of nocturnal migration would long ago have 

 been formed, and it would, by inheritance, be continued 

 by their descendants, even after the enemy had been long 

 extinct. 



Having reached their northern summer homes, and, 

 free from molestation, reared their broods, clearly, if all 

 things needed for their comfort were to be obtained, it 

 can not be supposed that these same birds would unneces- 

 sarily retrace their long flight to the distant south. This 

 suggests that, if I am correct in assuming that birds first 

 appeared in a tropical climate, and from such climate 

 migration started, it is probable that, by gradually pro- 

 longing their northern visits and accustoming themselves 

 to northern insect and vegetable life, these regions be- 

 came 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 southward those 

 farthest at the north. This accession of intense cold 

 necessarily decreases the amount of food, and the birds 

 are now forced to find it elsewhere. Farther and farther 

 south they come, just in advance of the cold, and slower 

 and slower they proceed as they enter our more temper- 

 ate 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 



