1592 THE STORY OF THE UNIVERSE 



must be remembered that a continuous tradition is 

 sustained; those who have made the journey before 

 guide the others. Doubtless they have memory for 

 great landmarks. They fly across the shallower parts 

 of the Mediterranean, where a chain of islands in this 

 submerged tract long remained to guide them. The 

 smaller birds usually keep nearer the ground; but it 

 must not be forgotten that the flight is usually mostly 

 accomplished by night. Birds generally meet in con- 

 courses, and migrate in flocks. Only a few fly alone. 

 Sometimes the old males remain, while the others 

 "flit." The return northward is more rapid, without 

 young ones or weaklings. The males often return 

 first. 



As birds have a full active life, with considerable 

 variety of function, in usually complex environment, 

 since, as we have already noticed, their sense-organs 

 and nervous systems are highly developed, consider- 

 able exhibition of intelligence is to be expected. 

 They seem to have great vividness of sense impres- 

 sions, to judge from their power of recalling old 

 haunts and old friends. Birds often return to the 

 same place season after season, and they have been 

 known to recognize an owner after the lapse of years. 

 Their quickness of ear and power of retention are 

 evidenced by the power some possess of learning to 

 repeat sounds, both words and tunes. Some have ex- 

 hibited marked fondness for music, and the aesthetic 

 tastes of the bower-bird excite deserved admiration. 

 Much more is known in regard to their marvelous 

 hereditary, general, and largely automatic reasonable 

 habits or "instincts" than in regard to their power of 



