380 THE STRUCTURE AND LIFE OF BIRDS cuap. 
the meeting-point will be, geologically speaking, soon 
reached. 
In the animal kingdom birds constitute what is 
called a class, while reptiles form another. A smaller 
division is called a sub-class, and below that, dividing 
and subdividing, we have orders and sub-orders, 
families and sub-families, genera, species, varieties and 
sub-varieties. The last two are different in kind from 
all the other divisions. The differences which mark 
off one variety from another are considered not to be 
constant: a few generations hence its representa- 
tives may have lost their distinguishing characters, 
Any species may vary, but one that is worthy of the 
name has its characteristic features so far fixed, that it 
is not likely at any near date to change in a way that 
might be inconvenient to our systems of classifica- 
tion. 
These divisions and subdivisions appear, at first 
sight, like complications. They are really a great 
assistance. Without them the field of ornithology, 
extending, as it does, over eleven thousand known 
species and some still unknown, would be a realm of 
chaos. The classification of birds has presented 
greater difficulties than that of any other class of 
animals. When the first attempts were made, wrong 
principles were adopted; more importance was 
attributed to habits than to structure. And thus 
such names as “Scratchers,” “Cooers,” “Waders” 
“Swimmers,” were given to various groups. Even 
now, in accordance with this wrong principle, the 
Swallow is sometimes put down as a near ally of the 
the Swift. In every case the aim should be to dis- 
