92 DISTEIBUTION OF ANIMALS. [part i. 



BIEDS. 



Birds are perhaps the most difficult to classify of all the 

 divisions of the vertebrata. The species and genera are 

 exceedingly numerous, and there is such a great uniformity 

 in general structure and even in the details of external form, 

 that it is exceedingly difficult to find characters by which orders 

 and families can be characterised. For a long time the system 

 of Vigors and Swainson was followed ; but this wholly ignored 

 anatomical characters and in many cases plainly violated well- 

 marked affinities. Characters derived from the form of the 

 sternum, the scutellation of the tarsi, and the arrangement of 

 the feathers, have all assisted in determining natural groups. 

 More recently Professor Huxley has applied the variations of the 

 bony palate to the general arrangement of birds ; and still more 

 recently Professor Garrod has studied certain leg-muscles for 

 the same purpose. The condition of the young as regards 

 plumage, and even the form, texture, and coloration of the egg, 

 have also been applied to solve doubtful cases of affinity ; yet 

 the problem is not settled, and it will probably remain for 

 another generation of ornithologists to determine with any 

 approach to accuracy what are the most natural divisions of the 

 class into orders and families. In a work like the present it is 

 evidently not advisable to adopt all the recent classifications ; 

 since experience has shown that no arrangement in which one 

 set of characters is mainly relied on, long holds its ground. 

 Such modifications of the old system as seem to be well 

 established will be adopted ; but the older groups will be ad- 

 hered to in cases where the most recent classifications are open 

 to doubt, or seem inconvenient as separating families, which, 

 owing to their similarity in general structure, form and habits 

 are best kept together for the purposes of geographical dis- 

 tribution. 



The old plan of putting the birds of prey at the head of the 

 class, is now almost wholly given up ; both because they are not 



