CAUSE AND ORIGIN OF MIGRATION 21 



Surely the question of original home, at anyrate 

 of the home in pre-Glacial days, may be entirely 

 left out of the question. No one can ever prove 

 that this wonderful memory did or could exist. 

 Post-Glacial dispersal northwards, and the founda- 

 tion of migratory habits of advancing to the new 

 food-producing areas, suitable also for the rearing 

 of young, was doubtless a fact, but would have 

 taken place in any case. The congestion due to the 

 increased numbers driven to a restricted area, 

 would involve a rebound outwards, and the unin- 

 habited areas northward of the refuge would be 

 the natural bourn towards which the birds would 

 travel. The seasonal return of cold would drive 

 them southwards in winter, and the periodical 

 migration habit would thus be originated. 



The intense love of home during the spread of 

 glacial conditions would tend rather towards ex- 

 tinction than the formation of any new habits. 

 The birds which possessed the greatest attachment 

 to the particular district would be less likely to fly 

 from adverse conditions, and the reduction of their 

 numbers through the ordinary physiological changes 

 in habit reduction of the number of young pro- 

 duced, and possibly disinclination to pair would 

 inevitably end in extinction. The stronger the 

 attachment to home the more likely the bird to 

 remain to the bitter end, and if driven away by 



