THESES FROM LITERATURE. 



1. In the Northern Hemisphere all migratory Birds breed on the 

 northernmost limit of their range; except in the Southern Hemi- 

 sphere, where every Bird breeds in a colder climate than that 

 in which it lives during migration at other times. 



2. The higher north Birds wander in summer, the farther south 

 they go for hibernation. 



Gatke, H.: „Die Vogelwarte Helgoland". Braunschweig 1891. 



1. There is a spring Migration, characterized by: 



a) want of disposition for more convenient routes, 



b) want of inclination to make a halt, 



c) restlesness and haste to proceed: 

 The cause is sexual instinct. 



2. There is an autumn Migration, but without the characteristic 

 features of spring Migration. 



3. in both seasons the routes are not the same. 



4. Under normal circumstances Migration is not observable. 



5. Disturbing influences — weather — allow fragments of the 

 Phenomenon to be observed. 



6. The flight during Migration is rapid, up to 53 geographical 

 miles an hour. 



7. The flight in Migration can take place at an altitude of 25000 

 to 35000 feet. 



8. Birdschoose those layersof air which are most convenient for mig- 

 ration ; as well as those currents of air which are most appropriate. 



9. This shows great sensitiveness (v. 8). 



10. Guidance by old individuals does not exist. 



11. A separate Migration -order according to age exists, viz. 



a) in spring the old ones arrive first, 



b) in autumn young Birds commence Migration. 



12. There are no Migration-routes but only Migration-directions- 



13. Migration did not develop from either straggling movement, 

 nor from heredity. 



(za 4.^ issa 



