156 ANIMALS OF THE PAST 



population by accident in recent times and has 

 also retarded the arrival of man. 



Once established, flightlessness and size play 

 into one another's hands; the flightless bird 

 has no limit placed on its size * while granted 

 a food supply and immunity from man ; the 

 larger the bird the less the necessity for wings 

 to escape from four-footed foes. So long as 

 the climate was favorable and man absent, the 

 big, clumsy bird might thrive, but upon the 

 coming of man, or in the face of any unfavor- 

 able change of climate, he would be at a se- 

 rious disadvantage and hence whenever either 

 of these two factors has been brought to bear 

 against them the feathered giants have van- 

 ished. 



REFERENCES 



' There is a Jine collection of mounted skeletons of va- 

 rious species of Mpas in the Museum of Comparative 

 Zoology at Cambridge, Mass., and another in the Amer- 

 ican Museum of Natural History, New York. A few 



* While we do not know the limit of size to a flying creature, 

 none has as yet been found whose wings would spread over 

 twenty feet from tip to tip, and it is evident that wings larger 

 than this would demand great strength for their manipulation. 



