The Birds, excluding the extinct form with teeth and a jointed tail, to which the group name of Archæornithes is given, fall into two groups. These are the Ratitæ, or Birds with Raft-like, _i.e._ flat, breast-bones, and the Carinatæ, or Birds with keeled breast-bones. The former include the African Ostrich (_Struthio_), the American Ostrich (_Rhea_), the Australian Emu, the Cassowary of New Guinea, and the Kiwi, or Apteryx of New Zealand; all of them birds that cannot fly. The vast majority of birds belong to the Carinatæ, characterised by the projecting keel (Carina) in the middle of the breast-bone. The presence of this, which affords a safe attachment for strong muscles, is associated with the power of flight. It is impossible to treat the birds more fully in the space allotted to this little story, but a few words about feathers, however, may find a place here. The colour of feathers is a subject of much interest. Everyone is familiar with the brilliant tints often presented by the feathers of birds, and everyone who is a close observer of natural objects knows that there are some feathers which are iridescent, changing colour according to the direction in which light falls on them. It has been shown by Dr. Gadow that this variation of the colour of a feather is due to its structure; this may be described as prismatic, for the small divisions of the feather present acute angular edges, which reflect the light like the edges of a prism. These are symmetrically repeated all along the feathers, so as to reflect the same colour throughout. Thus in the plumage of the common red and green parrot, we see feathers that are red when held in one position, and yellow when shifted to another position; while there are also feathers that are blue when seen in one position, and green when seen in another; the alternative colour being the one next in order in the rainbow.