IOO Allen, Origin and Distribution of N. A. Birds. f April 



fragile nature of most bird skeletons, and the small size of the 

 greater part of the species, and the fact that only those of more or 

 less aquatic habits would be liable to rapid entombment under 

 conditions favorable for their preservation, are circumstances 

 which render hopeless any expectation of the future discovery of 

 the ancestral lines of the great bulk of our present avian fauna. 



Two hypotheses, however, may be hazarded regarding the 

 present limited distribution of many groups now restricted within 

 comparatively small areas. First, that they had not only a local 

 origin, but that for some not very obvious reason they have always 

 had a local distribution, as for example the Todies in the West 

 Indies, and various South American and tropical Old World 

 types. Second, that they have become specialized since the close 

 of the Tertiary, with adaptations to a tropical or semi-tropical 

 environment. 



There is evidence that towards the close of the Tertiary a 

 marked change in the earth's climate took place, culminating in 

 the Glacial Period, during which the whole northern half of the 

 northern hemisphere became covered with a heavy ice cap, 

 lasting for possibly thousands of centuries, and extending its 

 chilling influence nearly to the northern tropic. The rise of the 

 o-lacial period was of course gradual, and the southward progress 

 of the great ice cap drove before it all foj"ms of life capable of any 

 considerable power of locomotion, while those unable thus to 

 escape must have perished from cold. There was hence a great 

 crowding together of exiles from the north into the more favored 

 regions to the southward, leading to an intense struggle for exist- 

 ence, and a weeding out on a grand scale of forms least fitted for 

 the contest. This period must thus have been one of great ac- 

 tivity in the evolution of new types. Opportunity was given for 

 the gradual adaptation of many forms to a lower temperature 

 than that to which they had been accustomed, and to an enforced 

 change of food. The recession of the ice fields was accompanied 

 by the gradual extension northward of habitable land. A broader 

 area becoming available in summer than in winter an annual 

 migration for a greater or less distance on the part of the pioneer 

 life became a necessity. Finally the ice receded to its present 

 limits and the whole north, under radically altered climatic con- 

 ditions, became again available for occupation by the more or 

 less modified descendants of the pre-glacial exiles. To some of 



