No. 568] SHORTER ARTICLES AND DISCUSSION 



253 



Classification of Barriers to Species as Regards 

 Birds and Mammals 



Barriers: 



A. Intangible. 



(a') Zonal (by temperature). 



(&') Faunal (by atmospheric humidity). 



(c') Assoeiational. 



(1) By food supply. 



(2) By breeding places. 



(3) By temporary refuges. 



(Each of these three with regard to the inher- 

 ent structural characters of each species 

 concerned.) 



B. Tangible (mechanical). 



{a") Land to aquatic species. 



(6") Bodies or streams of water to terrestrial species. 



The above categories are believed to include all the factors 

 commonly involved in checking the spread of species of birds and 

 mammals. It is possible that inter-specific competition may 

 sometimes occur where assoeiational homologues meet. But even 

 here it becomes a matter of relative assoeiational fitness which 

 determines supremacy and consequent ultimate limits of inva- 

 sion of the forms concerned. 



A mountain range. mech.-miiMlly sprnking. 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 



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

 ence 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 analysis here presented. And as de- 

 tailed knowledge of the facts of geographical distribution accu- 

 mulates, the delimiting factors become more and more readily 

 detectable. By such a study, of comparative distribution, it 

 seems possible that the ranges of birds and mammals may become 

 subject to satisfactory explanation. 



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 multiplication of species, in birds and mammals, brought 

 about. 



