THE GLACIAL RANGE CONTRACTION, ETC. 6i 



occupied by the species or its ancestral forms, or the 

 remnants of such that escaped extermination during the 

 Glacial Epoch, however near or remote such Range 

 Bases or Refuge Areas may be from their present 

 breeding grounds. 



Corollary II. — The northern limit of a Northern 

 Hemisphere species in any area marks the limit of Emi- 

 gration from a more southern area, not necessarily south 

 of it. A bird's Breeding Grounds are always at the limits 

 either of its Past Emigrations or Present Migrations. 



Corollary III. — The Present Migration of a species 

 is a recapitulation of the Past Emigrations or Range 

 Expansion of that species. 



Corollary IV. — Species never *' retreat " from ad- 

 verse conditions. If overtaken by such they perish, or 

 .such portion of the species that may be exposed to them. 



Corollary V. — Extension of Range is only under- 

 taken to increase Breeding Area, and therefore during 

 most favourable climatic conditions. 



Corollary VI. — Contraction of Range is only pro- 

 duced by Extermination amongst sedentary species, and 

 probably also by Extermination (through inability to 

 rear offspring) amongst migratory species that are 

 neither Inter-polar nor Inter-hemisphere. 



In my opinion this Law will explain not only why no 

 typical Nearctic species encroach upon the Palsearctic 

 Region across Behring Strait,^ but also assist in solving 



^ I am fully aware that there is a slight intermixture of Patearctic 

 and Nearctic species in Alaska and North-eastern Siberia, especi- 

 ally amongst migratory birds. I have already alluded to this fact 

 in the Migration of Birds (p. 234) ; whilst JNIr. Seebohm has 

 recently pointed out other instances (conf. Ibis, 1894, p. 293). 

 There is, however, not the slightest evidence to suggest that any 



