MIGRATION. 167 



of its rays they hasten to their own destruction we can also 

 see by looking at an electric light on a foggy evening, seeing 

 the halo it builds out upon the mist and the solid pencils of 

 light that stream out from it. A bird striking one of these 

 beams of light and seeing nothing beyond but a blank wall of 

 darkness, dazzled and bewildered, follows up the ray in which 

 he is confined, a cage of light that he cannot break out from, 

 until he dashes against the lantern. 



What birds migrate by day and what by night and why 

 they differ in their habits, is another interesting problem of 

 migration, and those who wish to study further will find in 

 the appendix Mr. Brewster's list and his conclusions, reached 

 after twenty years of study, which show that not even in 

 selecting a time to travel do the birds act without good 

 reasons. 



But another point much more likely to attract our attention 

 is the way this army is guided, why the birds all go at one 

 time, and how those born in a northern home can find their 

 way thousands of miles to places they have never seen. We 

 shall find that they do not all start at one time, as we com- 

 monly think, but begin to slip away weeks before we take 

 notice of their departure, the places of those we have been 

 acquainted with in our gardens being filled with strangers 

 from the north. About the earliest to leave in a body are 

 the swallows, the dates of whose departure are easily deter- 

 mined, but the others are passing and passing for many 

 weeks in a leisurely procession. We cannot give a better 

 idea of the way the migration is accomplished than by quot- 

 ing again from Mr. Brewster : 



" The conditions which cause one flock, or family, or individ- 

 ual to start southward are ordinarily so widespread and gen- 

 erally operative, that countless flocks, families, and individuals 



