SUMMARY AND CONCLUSION 289 



Glacial Epoch in that hemisphere the Southern Hemi- 

 sphere would most likely present the most favourable 

 conditions for successful reproduction, if land extended 

 southwards. Thus from a common equatorial range 

 base during the course of ages, would the breeding area 

 of these Inter-hemisphere and Inter-polar species be to 

 a varying extent reversed in the former group, and 

 completely reversed in the latter, the breeding grounds 

 in one hemisphere becoming the winter quarters in the 

 other (conf. Migration of Birds, pp. 149-152). On the 

 other hand, Northern Hemisphere species living per- 

 manently in the northern areas, with no southern breed- 

 ing range base beyond the limits or fatal influence of 

 glaciation, would gradually be exterminated; or Southern 

 Hemisphere species living permanently in the southern 

 areas with no northern range base beyond the limits or 

 fatal influence of glaciation would also be as surely 

 exterminated, because the Law governing their dis- 

 persal forbids retreat. The significance of these facts, 

 and their bearing on the Arctic element in South 

 Temperate floras, I hope presently to show. 



We then proceeded to apply this Law of Dispersal to 

 the glacial range contraction and Post-Glacial emigra- 

 tion of British birds, and endeavoured to show how 

 existing species were preserved during the Glacial 

 Epoch ; how such surviving forms emigrated or ex- 

 panded their range north with the return of more 

 favourable climatic conditions. Passing, then, from a 

 study of universal emigration during remote ages to 

 more local range expansion actually in progress at the 

 present time, we proceeded to show that the movement 



is governed by precisely the same law that controlled it 



U 



