4o6 A HISTORY OF BIRDS 



that it is made up of a tissue of a " mucous " character such as 

 is met with elsewhere only in the umbilical cord connecting the 

 mammalian foetus with the parent. So far the purpose of these 

 bodies is quite unknown. 



No less puzzling is the median septum which runs up the 

 middle of the tracheal tube in birds so widely distinct as 

 Humming-birds and Petrels, Spoon-bills and Penguins. Possibly 

 it is the last vestige of a sometime double trachea which has 

 now fused, inasmuch as a similar septum is found in all embryonic 

 birds. This, except in the cases just quoted, becomes com- 

 pletely absorbed, save only that portion which remains to form 

 the "pessulus ". 



There are other similar instances of structures which appear 

 to be preserved in a high state of efficiency, but whose functions 

 are so far unknown, or but imperfectly understood : but, in so far 

 as birds are concerned, the modifications of the windpipe are 

 the most striking. 



What may be called the principle of reciprocity in develop- 

 ment is nowhere more perfectly illustrated than in the case 

 of the tunnelling of the keel of the sternum by the windpipe, 

 or by the cup-shaped receptacle formed, it is assumed, by the 

 pressure of the windpipe on the symphysis of the clavicle, in 

 the case of the Guinea-fowls of the Genus Guttera. The pheno- 

 mena here exhibited appear hitherto to have been passed without 

 comment, yet it is certainly remarkable that this keel, elsewhere 

 among birds a thin plate of bone, should in these Swans and 

 Cranes have developed into a cavernous receptacle, involving 

 also the body of the sternum forming the roof of this space. 

 What are the factors which have brought about the formation 

 of this curious chamber ? The theory of natural selection here 

 seems inadequate ; while the theory of mutation involving the 

 sympathetic variation of two such dissimilar structures seems no 

 more helpful. 



For the capture of ants and other insects the Woodpeckers 

 have developed a protrusible tongue — agreeing therein with 

 many ant-eating mammalia — and powerful salivary glands. 

 Yet, without any such elaborate apparatus one of the King- 

 fishers {^Halcyon cyanoleucus) of Liberia, which, after the fashion 

 of his people, is almost tongueless, contrives to subsist on ants 

 without any special apparatus for their capture ! 



