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and various theories have been put forward in explanation, but 

 we may fairly assume that the pioneers of these latter movements 

 were only driven to undertake such a perilous flight under 

 pressure of urgent need. It seems highly probable that accident, 

 or at any rate meteorological conditions had a great influence on 

 the ultimate bourne at which these first flights arrived. No 

 doubt numbers perished, as they do even at the present day, 

 but still sufficient survived, who were impelled by the love of 

 their old haunts to retrace their steps when the conditions had 

 ameliorated. The theories of land-bridges, old coast lines, or 

 groups of islands, since submerged by geological changes, have 

 been put forward by certain ornithologists as explanatory theories 

 of the manner by which this habit of crossing wide seas has been 

 acquired. No doubt there is much to be said in their favour, and 

 that their influence on the path of migration is still traceable in 

 various localities at the present day. 



Emigration, or the extension of the breeding range of species 

 has also had its influence on migration. Species have extended 

 in various directions, some from the south, others from the south- 

 east, or even east. Those from the south will have thus met the 

 cold in its advance, or followed it in its retreat, but the manner 

 in which they have acquired the habit of migration will not be 

 essentially dissimilar to that which has already been described. 

 Nevertheless, there are cases in which birds migrate from their 

 ancestral home in the south to breed in the north. According to 

 Seebohm the Swallows are an example of the latter phenomenon. 



It now remains to add a few remarks on the evidence in 

 favour of the possession by birds of an acute sense of direction 

 and locality. It has been previously pointed out that so far from 

 enunciating any theory of his own as to " What guides birds 

 during their migrations," Herr Gatke frankly avows himself dis- 

 inclined even to undertake the task. The chapter in his work 

 he devotes to the question is, therefore, nothing but a statement 

 of his objections to the theories of others ; to only one of which 

 does he appear to give even partial adherence. 



It is, perhaps, not to be wondered that he should feel dis- 

 inclined to put forth any theory of his own, committed as he 

 already is to the statements on the direction, altitude and 



