BIRDS AND LIGHTHOUSES 271 



influences the departure in the autumn ; the third 

 of these which provokes the return in spring ; while 

 the second may be allowed to have an important 

 bearing on the movements of birds at all seasons. 



In making this statement, it should be observed 

 that I am not putting forth the views enunciated by 

 any particular writers to whom I have referred, but 

 am rather stating the conclusions at which I have 

 arrived after some consideration of the subject, and 

 a perusal of what they and others have written. 



As regards the faculty by which migration is 

 performed, it is much more difficult to express an 

 opinion, although I might give expression to the 

 views of others who are much better able than I 

 am to form a judgment in the matter. Before 

 coming to any conclusion on this point, however, 

 it seems desirable to consider first the probable 

 origin of the instinct of migration. 



It is not unreasonable to assume that the change 

 in climate at the close of the Tertiary period, which 

 reduced the temperature of the higher latitudes from 

 warm temperate to frigid conditions, must have 

 resulted in the crowding of bird-life towards the 

 equator, thereby intensifying the struggle for ex- 

 istence to such a degree, that overcrowding would 

 induce those species best able to withstand climatic 

 change to avail themselves of the milder interim of 

 summer to enlarge the boundaries of their range, 

 while the recurrence of winter would force a. 

 temporary removal to milder regions.^ 



1 I have here adopted the views, in his own words, of a well- 

 known American ornithologist, Mr J. J. Allen. 



