110 University of California Publications i)i Zoology [Vol. 12 



The above categories are believed to include all the factors com- 

 monly involved in the checking of the spread of species of birds and 

 mammals. It is possible that inter-specific competition may sometimes 

 occur where associations! homologues meet. But even here it becomes 

 a matter of relative associational fitness which determines supremacy 

 and consecpient ultimate limits of invasion of the forms concerned. 



A mountain range is no barrier at all, per se, as frequently alleged. 

 Only as it involves zonal or faunal barriers does it affect distribution. 

 The same is true of a valley or a desert. 



As far as contemplation of cases has gone, the writer's experience 

 has led him to believe that the outlines of the ranges of all birds 

 and mammals may be accounted for by one or more of the factors 

 indicated in the above analysis. And as detailed knowledge of the 

 facts of geographical distribution accumulates, the delimiting factors 

 become more and more readily detectable. By such a study, of com- 

 parative distribution, it seems possible that the ranges of birds and 

 mammals may become subject to satisfactory explanation. The 

 instances included in the list of species discussed in the present paper, 

 when considered in connection with many similar ones, point without 

 exception towards the existence of the set of factors above specified 

 as delimitors. 



When considered in its historical bearing, the problem of barriers 

 concerns itself intimately with the origin of species. It is believed by 

 the writer that only through the agency of barriers is the multiplica- 

 tion of species, in birds and mammals, brought about. 



CHECK-LIST OF THE BIRDS 



1. Gavia immer (Briinnich) 



2. Sterna forsteri Nuttall 



3. Phalaerocorax auritus albociliatus Ridgway 



4. Pelecanus erythrorhynckos Gmelin 



5. Mergus serrator Linnaeus 



6. Anas platyrhyncbos Linnaeus 



7. Nettion carolinense (Gmelin) 



8. Querquedula cyanoptera (Vieillot) 



9. Spatula clypeata (Linnaeus) 



10. Dafila acuta (Linnaeus) 



11. Marila affinis (Eyton) 



12. Erismatura jamaicensis (Gmelin) 



13. Chen hyperboreus hyperboreus (Pallas) 



14. Plegadis guarauna (Linnaeus) 



15. Mycteria americana Linnaeus 



