that their bones have thin walls, the large cavity of which in all probability contained air-sacs. Even some of the jumping Dinosaurs, to which reference has already been made, seem to have possibly had lungs constructed on the bird type. We see, therefore, that even where a bird is, so to speak, most characteristically a bird--in the subsidiary mechanisms of flight--it betrays a likeness to the comparatively grovelling reptile, letting alone the aërial and more bird-like Pterodactyles. Brain. The brain of birds is large in proportion to the body, thus contrasting with that of the unintelligent reptile. From some tables on the matter which have been published, it appears that, if weight of brain goes for anything, the goldfinch is one of the most intelligent of birds. The weight of its brain is one-fourteenth of the entire weight of the body. The most unintelligent of all is the domestic fowl, whose body is 412 times heavier than its brain. The size of brain, however, seems to be largely a matter of the size of the bird: generally speaking, the smaller birds have heavier brains, and _vice versâ_. One might have expected something from the apparently intelligent Parrot; but the brain of the ‘Amazon’ is only one forty-second part of the weight of its body. Even the cruel and bloodthirsty Hawk, which one associates with brutality and ignorance, has a brain which is but little heavier. The front part of the organ, known as the cerebral hemispheres, or, more briefly, as the cerebrum, is that part of the brain which is associated with intelligence. Now among the mammals this part of the brain is generally much furrowed, the brain surface being, therefore, increased without any actual increase in the skull-space required. This furrowing is met with in most mammals, but not always in the smaller and in the less intelligent kinds. But in the bird’s brain there are no convolutions: the surface is as smooth as in the reptile. Not even in the artful Raven, which some hold as the most highly developed of birds, is there a trace of the furrowing which one rightly associates, so far as the mammalia are concerned, with a high position in the series. The hinder part of the brain is known as the cerebellum; between this and the cerebrum are the optic lobes, of which there are only two, the mammals having four. From the brain arises the spinal cord, or marrow, which runs in the canal formed by the vertebræ, just as the brain lies in the brain-case. The nerves of the body come off either from the brain or the marrow, but it is not important to enumerate them. They show no difference in different kinds of birds. The Muscles.