Birds, as regards structure, are perhaps the most perfectly endowed, as they are certainly the most beautiful and interesting, of all the lower animals. In Birds there is an admirable mechanism and adaptation both for gliding in the air and swimming in the water. They surpass all other animals in the faculty of continuing their motion without resting, as well as in its rapidity. The fleetest courser can scarcely ever run more than a mile in a minute, nor support that speed beyond five or six such exertions. But the joyous Swallow does this tenfold for pleasure. In his usual way he flies at the rate of one mile in a minute; and Wilson, the ornithologist, ascertained that the Swallow is so engaged for ten hours every day. So can the Blue-bird of America, for a space of 600 miles. Our Carrier-pigeons move with half that celerity: one flew from Liskeard to London, 220 miles, in six hours. The Golden Eagle is supposed to dart through the fiercest storm at the rate of 160 miles an hour; but one of our smallest Birds, the Swift, can even quadruple the most excited quickness of the race-horse for a distance. Spallanzani thought that the little Swift travelled at the rate of 250 miles an hour. Inquiries into the phenomena of the flight of Birds would lead us far beyond our limits. The subject is beset with error. Thus, we read:--"Every one has remarked the manner in which Birds of prey float, as it were, without any effort, and with steady expanded wings, at great heights in the atmosphere. This they are enabled to do from the quantity of air contained in the air-cells of their bodies, which air being taken in at a low level in the atmosphere, of course rarefies and expands as the Bird ascends into higher regions. Their rapidity of descent must be accomplished by the sudden expulsion of this air, aided by their muscular efforts." Now, Dr. Crisp has read to the Zoological Society a paper "On the Presence or Absence of Air in the Bones of Birds," for the purpose of showing the prevailing error upon the subject--viz., "that the bones of the Bird are filled with air." Of fifty-two British Birds recently dissected by him, only one, the Sparrow-hawk, had the bones generally perforated for the admission of air. In thirteen others, the humeri only were hollow, and among these were several Birds of short flight. In the remaining thirty-eight, neither the _humeri_ nor _femora_ contained air, although in this list were several Birds of passage and of rapid flight--Dr. Crisp's conclusion being, that the majority of British Birds have no air in their bones, and that, with the exception of the Falcons, but very few British Birds have hollow femora.