124 



NATURAL HISTORY OF BIRDS. 



section of it shows it to consist of two thin walls, separated by a marrow-like substance. 

 In this feature of the sternum we see the only iu.licutiou in the embryo of the singular 

 structure to be developed later in life. The degree of complexity of the trachea is 

 thus shown to be dependent upon age, and the variations are no doubt fully accounted 



for by this fact." 



The cranes are gregarious, and those inhabiting the northern hemisphere are emi- 

 nently mi-'ratory. Durintr their migrations the flocks travel in V-like array, like those 

 of wild geese. They are mainly vegetable-feeders, and some species are even exceed- 

 indy destructive to the grain crop. 



^ The -roup is one of considerable anti-iuity, and w.as formerly richer m forms than 

 nowaday, like the next foregoing and following f.amilies. A gigantic species, Grua 



^ 



Fio. 59. — Brcaat-bono and lower part of windpipe of the whoo^iing crane, Grtts amfrictttn. The entire left side of 

 the keel has been cut away to sliow the interior. About one-balf natural size. 



primigenia, inliabited France during the ' reindeer period,' and remains of cranes have 

 been traced back to the miocene of Europe and the pliocene of North America. At 

 present the species are few, and their geographical distribution somewhat ])eculiar. 

 They occur now, during one or another season of the year, everywhere except in 

 South America and the Malayan and Polynesian archipelagoes. One species is 

 peculiar to Australia, two are North American, four arc exclusively African, while 

 the rest arc chiefly Asiatic; the headquarters of the restricted genus Grt(s being the 

 tem])erate parts of eastern Asia. 



Altogether there are only .about seventeen welklefined species, referable to three 

 or four genera. On the full-page plate the three chief forms are represented. In the 

 background are seen a flock of the common European crane ( Grus </rus), yelling at 



