THESES FROM LITERATURE. 



7. An interchange of individuals does not take place on the route. 



8. Over unfavourable districts Birds pass without break. 



9. The total number of the individuals depends on the frequency 

 of the species at the starting point and on the nature of the route. 



10. Most of the Birds do not fall under the same category for the 

 breeding-season as for Migration. 



1 1. The departure may be simultaneous; in the procession the in- 

 dividuals have the same rank, as in the breeding-zone: succes- 

 sive migration; or the northern ones set out earlier: passage. 



12. Hereditary organisation makes Migration possible, but does not 

 cause it. 



13. The migratory instinctdepends partly on bodily, partly on traditio- 

 nal heredity. 



14. The knowledge of the Migration-routes becomes traditional with 

 the species. 



1 5. The beginning of Migration originates in irregular passing. 



16. The appearance of stragglers is the result of Migration departing 

 from the routes. 



17. If stragglers breed in strange countries, the geographical limits of 

 the species are extended ; regular Migrations leave them as 

 they are. 



18. This phenomenon explainesabbreviated and protracted migration. 

 IQ. Migration-routes may also give an impulse to evolution of forms; 



such arise from irregular Migration. 



' On Method. 



20. Dates of arrival scarcely give sure results, they are most critical 

 to work up. 



21 . Dates from different places are only commensurable, if they are 

 of the same year. 



22. A typical series is a uninterrupted series of epochs drawn from 

 mean data. 



Ko 39 iss 



