The opening of Introduction
DEFINITION.–"A Bird is a feathered biped." This popular saying undoubtedly furnishes a definition in the world of to-day, since no other existing creature has a clothing of feathers, and even the word "biped" is thus superfluous.
The above should, however, be somewhat expanded, in order to shew in greater detail the differences between Birds and other Vertebrata. Care must nevertheless be taken to avoid the fault common to many modern definitions, of giving an abstract of the main characteristics of the object, rather than a clear guide to distinction.
Dr. Gadow[1] defines Birds as "oviparous, warm-blooded, amniotic Vertebrates, which have their anterior extremities transformed into wings. Metacarpus and fingers carrying feathers or quills. With an intertarsal joint. Not more than four toes, of which the first is the hallux."