THESES FROM LITERATURE. 



10. Migration, owing to which birds enjoy the benefitsof this two- 

 fold summer, leads them from the Arctic regions to the southern 

 parts of the Continents and vice versa. 



1 1 . Tropical Birds do not migrate atall,oronly in a very limited degree. 



12. The objectofthe greatest part of northward— bound Migrants is 

 breeding; of those passing to the south, food, warmth and light. 



13. The breeding place is not necessarily the original home of 

 the species. 



1 4. It is not always the breeding regions that have the coldest climate. 



15. As a rule, species or individuals which migrate far to the 

 North, migrate far south too. 



16. it needs a long time for Migration to effect a differentiation 

 of species; less time is sufficient to produce more or less modi- 

 fication of Migration. 



17. Migrating companies of one and the same species may travel 

 in various directions, the routes may even cross. 



18. The sense of orientation is not infallible, but develops with 

 age and improves. 



19. The younger generation is guided by the old Birds, which 

 know the route by experience. 



20. If the younger ones by chance lose the proper route, they 

 have much difficulty in finding it again, hence the fact that most 

 of the stragglers are young Birds. 



Anoot, a.: „Annales du Bureau-Central Meteorologique" Paris 1895. 

 1. Temperature is not an incentive to Migration. 



Newton, A. Prof.: sets forth the hypothesis of over-crowding; 

 arriving migrants replace those which had arrived before, there 

 being no room for both in the same place (Cfr. Gatke : „Heli- 

 goland as an ornithol. observatory" 1895. p. 145 and the new 

 „Naumann" ed. by Dr C. Hennicke. Gera, part I. 1905 p. 

 101. and W. RuSKlN Butterfield„Remarks upon . . . Migration, 

 of Birds". Nov. Zoological Vol. Xll. 1905. p. 16.). 



tK 47 120 



