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 indication 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 migratory. During 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- 

 ingly destructive to the grain crop. 



The group is one of considerable antiquity, and was formerly richer in forms than 

 nowadays, like the next foregoing and following families. A gigantic species, Grus 



FIG. 59. Breast-bone and lower part of windpipe of the whooping crane, Grus americana. The entire left side of 

 the keel has been cut away to show the interior. About one-half natural size. 



primigenia, inhabited 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 peculiar. 

 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 are exclusively African, while 

 the rest are chiefly Asiatic ; the headquarters of the restricted genus Grus being the 

 temperate parts of eastern Asia. 



Altogether there are only about seventeen well-defined 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 grus), yelling at 



