12 CALIFORNIA ACADEMY OF SCIENCES. 



procreation in migratory birds is doubtless a stimulus in 

 the return-journey. That there is, however, above and 

 behind this a deeper incentive, is manifested through early 

 summer migration southward, as in the California Murre. 

 The physical necessity exists for alternate depopulation 

 and repopulation of the Northern and Southern Hemi- 

 spheres as winter holds the supremacy in either. The 

 adaptation of birds to this physical necessity is migration. 

 Two prime factors enter into the adaptation — the past 

 and the present. That the inheritance from the past 

 amounts to anything more than the immediate urgency 

 requires, the facts of migration apparently do not evidence. 

 While migration may have originated with the Glacial 

 Period or more remote secular refrigeration, and have 

 been the adjustment to the conditions that existed then, 

 the present adjustment meets the conditions of winter as 

 they exist now. In brief, it is maintained that all the 

 diverse movements constituting migration are the out- 

 growth of time and are enforced by the present need of 

 adjustment of population to food-supply.* On one hand 

 there is intelligent t adaptation on the part of the birds 

 to the present necessity for migration, on the other hand 

 there is probably a hereditary disposition for travel 

 awakened into migration in the young by the example of 

 the adults — the habit, acquired in youth, becoming fixed 

 to retreat after reproduction along familiar routes to re- 

 gions where food will not fail and afterward to return to 



* So nice is this adjustment between population and food-supply that 

 birds sometimes perish for want of food through extraordinary severity 

 of season (see 'Auk,' vol. xi, pp. 110, 111; vol. xii, Smith, p. 1S3; Wayne, 

 p. 184; Cory, p. 187; Deane, p. 303). 



t The overcoming of obstacles by the adults when the young succumb, 

 the meeting of exceptional conditions by exceptional migration, the ap- 

 parent recognition of landmarks, and the exercising of leadership, appear 

 to warrant this statement. 



