THESES FROM LITERATURE. 



2 The cause of Migration is the search for plentiful food. 



3 A period of 2000 years is sufficient to estabhsii Migration. 



4. There are some exceptions to Mr. Seebohm's law that „every bird 

 breeds in the coldest regions of its Migrations". („Siberie en 

 Europe" p. 244.) 



5. The country in which a migratory bird breeds is not necessarily 

 the home of the species. 



HartwiQ: „Zum Vogelzuge". Journ. f. Ornith. 1885. 



1 Almost the chief reason is food ; temperature and light are of 



minor influence. 

 MenzbIER, M.: „Die Zugstrassen der Vogel im europaischen Russland". 



Bull, de la Soc. Imp. des Nat. de Moscou" Ann. 1886. No 2. 



1. Only twolargeCategoriesof Migration-routes can be accepted: 

 viae marinae littorales — and : viae subcontinentales; viae sub- 

 marinae littorales signify a transitory category. 



2. Every species travels its own way; what Biologists call Mig- 

 ration-routes are only the coincidence of the routes of several 

 species for a shorter or longer distance. 



3. The Phenomenon of Migration is called forth by the conditions 

 for the acquisition of food ; the Migration-routes originate in 

 the history of the range of the species and feeding and oro- 

 hydrographical conditions. 



4. The breeding-places have an influence on the Migration-routes 

 of the species, but not exclusively and not for all species; for 

 the majority of continental Birds a change in the breeding-places 

 must be considered as a normal phenomenon. 



5. The Migration-routes change in the course of time either in 

 connexion with changes in the distribution of the species: 

 or in coimexion with changes in the oro-geographical cha- 

 racter. 



6. The routes of the autumn and spring Migration are not always 

 the same ; in some cases the difference between spring and autumn 



BBi 43 ai 6* 



