2 BIRDS OF EASTERN NORTH AMERICA. 
largest Ostrich, which, it is claimed, was still in existence at the beginning of the 
present century, and a few of the older natives claim to have heard their fathers talk 
of seeing it alive. We know of several species which have become extinct during the 
past fifty years, notably the Great Auk and the Labrador Duck, Camptoleemus labra- 
dorius. 
In the West Indies we have numerous instances of the recent disappearance of 
insular forms. The Jamaica Petrel, Aestrelata caribwa, is now supposed to have been 
exterminated by the Mongoose (Herpestes), which animal was introduced into the 
island some years ago for the purpose of killing the rats. Wallace, in his ‘‘ Distri- 
bution of Animals,” states that no less than six species of Parrots were said to have 
been formerly found in Guadaloupe and Martinique, but at the present time no Parrots 
are known to exist on either of those islands. 
99 
Ledru, in his ‘* Voyage aux iles Teneriffe,” published in Paris, in 1810, men- 
tions several birds which are now unknown, among them a green pigeon from St. 
Thomas. 
While many species have become extinct, others have extended their range, and, 
accommodating themselves to changed conditions and environment, have in the course 
of time developed new forms. Birds showing decided and constant differences are 
recognized as species, whereas if the differences in color or size are not very great and 
intermediate forms occur showing an intergradation from one to the other, they arc 
called races or subspecies. Races are really species in process of development and 
are caused by difference in climate, food, etc. Insular forms which are sufliciently 
removed from the parent stock to warrant the belief that their isolation is complete, 
are generally recognized as species even though the differences would hardly be worthy 
of specific recognition if the two forms occurred in close proximity on the main land. 
It should be borne in mind, however, that the line of demarkation between a species 
and a subspecies is a purely arbitrary one and is largely a matter of individual opinion. 
Subspecies are distinguished by a third name; for example, Dendroica paimarum 
hypochrysea is & race or subspecies of Dendroica palmarum. 
Having learned something as to what a bird is, let us take up in order the more 
important external parts, such as the wing, tail, bill, and feet, 
