2 ELEMENTS OF ORNITHOLOGY. 
The Class of Birds stands out in startling contrast to all the 
other three Classes just referred to, and is one of the most 
definite, uniform, and easily defined groups (of its own rank) 
in the whole Animal Kingdom *. 
All Birds have feathers, and no such thing as a feather is 
possessed by any creature which is not a Bird. This character 
alone, therefore, suffices to define the whole Class. 
The uniformity of their structure is very remarkable. While 
some beasts, some reptiles, and some fishes have but a pair of 
limbs, and other reptiles and fishes have none, all Birds have 
two pairs—a pair of wings and a pair of legs. Most beasts and 
reptiles have a long tail, while some are without any; but 
every existing Bird has a tail which, feathers apart, is short +. 
Some beasts, some reptiles, and some fishes are edentulous, 
while most have teeth; but no living Bird possesses such 
structures. Some beasts, some reptiles, and some fishes are 
eyeless ; but every Bird has a pair of eyes. 
Birds are eminently aérial creatures, and there are but very 
few which have no power of flight; while none of the living 
members of the other Classes can truly fly except Bats; though 
some ancient reptiles (Pterodactyles) possessed a similar power. 
Although many birds are more or less aquatic in their habits, 
none are so completely so as either the Whales and Porpoises 
amongst beasts, or certain reptiles. 
Birds differ much in size, as, ¢.g., the Ostrich and the 
Wren; but the differences are not so great as those which 
exist between different beasts, different reptiles, and different 
fishes. 
No Bird, however cold a climate it may inhabit, ever falls 
into prolonged winter sleep (hibernates) as do some mammals 
(e.g., the Dormouse) and a number of reptiles. Many Birds 
avoid undue cold or heat by a periodical change of place, or 
“migration,” which is fixed and definite. Thus some Birds 
come to us in winter from the North, and more in summer from 
the South. 
Different kinds of Birds inhabit different regions of the 
earth’s surface, and their distribution is necessarily restricted 
by the supply of suitable food and other conditions needful for 
* All animals taken together are spoken of as the Animal Kingdom in 
contrast and distinction to the Vegetable Kingdom, which includes all 
plants. 
t In Chapter V. we shall see that this was not always the case. 
