24 SYLVAN SECRETS. 



ternary age there was a series of climatic 

 oscillations, the tropical temperature swaying 

 back and forth over a wide area from north 

 to south. The birds migrated to and fro un- 

 der the impulse of a natural desire to keep 

 within an agreeable habitat. These oscilla- 

 tions of temperature were on a large scale ; 

 but, from the nature of things, there were 

 intermediate disturbances of a like character, 

 and of far slighter eif ect. No doubt the birds 

 resisted these changes with stubborn persist- 

 ency, giving way before them only at the 

 last moment, and returning upon their haunts 

 with each temporary relaxation of the icy 

 grip, to be driven away again and again 

 through a long series of generations. This 

 struggle for the old northern home, kept up 

 for ages, became a hereditament of bird- 

 nature, an instinct, as we call it, a natural 

 desire, indeed, irresistible and perpetual. 

 The migratory birds are the old birds of the 

 north. With them the polar region is a dim 

 and tender memory transmitted from a re- 

 mote ancestral source. 



The non-migratory species are those birds 

 whose physiques were long ago so modified 

 that natural desire for a lost habitat was ex- 

 tinguished and equilibrium reached. 



The aquatic and semi-aquatic birds are 

 mostly very distant migrators, and yet, ap- 

 parently, they have the least need to migrate 

 at all. Why, for instance, should a Florida 

 gallinule leave the plashy, lily-lined margins 

 of the southern lakes in spring and go far 

 north to less eligible waters? Why do so 

 many wood duck, teal, snipe, herons and 

 bitterns come out of the South to breed? 

 The fact that many, very many, of these 

 birds do not migrate at all is strong proof, I 



