In some ways birds are the highest of the vertebrate animals. They
represent the climax of that passage from water to land which the
backboned series illustrates. Their skeleton is more modified from the
general type than that of mammals; their arrangements for locomotion,
breathing, and nutrition are certainly not less perfect; their body
temperature, higher than that of any other animals, is an index to the
intense activity of their general life; their habitual and adaptive
intelligence is familiarly great, while in range of emotion and sense
impressions they must be allowed the palm. It is, in fact, only when we
emphasize the development of the nervous system and the closeness of
connection between mother and offspring, that the mammals are seen to
have a right to their pre-eminence over birds.