33o THE STRUCTURE AND LIFE OF BIRDS CHAP. 



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- 



