2 ELEMENTS OF OENITHOLOGT. 



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 

 stlone, 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 f. 

 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 aerial 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 difier much in size, as, e. 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 vidnter 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 animalB taken together are spoken of as the Animal Kinedom 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. 



