92 | DISTRIBUTION OF ANIMALS. [PARY L. 
BIRDS. 
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 Prefessor 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 
