In the Python, Fig. 4, there are, near the central line, numerous round spots, which apparently emphasize the neural processes. There are diagonal markings on some species which illustrate the development of colour-spots already alluded to. This snake-pattern is very singular and striking. The markings are fewer in number than the vertebræ, yet their true vertebral character is most obvious. In Snakes, again, we find the dorsal region is darker than the ventral. In the Lizards there are patches of colour placed axially, while each patch covers a number of scales. _Birds._ Birds have their whole economy modified to subserve their great functional peculiarity of flight. Immense muscles are required for the downward stroke of the wing, and to give attachment to these the sternum has a strongly developed keel. To bring the centre of gravity low, even the muscles which raise the wing are attached to the sternum, or breastbone, instead of to the dorsal region, as might be expected; and to brace the wings back a strong furculum--the merry-thought--is attached. The breast, then, is the seat of the greatest functional activity in birds, and, consequently, we find in a vast number of birds that the breast is the seat of vivid colour. As many birds are modified for protective purposes, the brightest species were selected to test our views, namely, the Birds of Paradise (Paradisea), Humming Birds (Trochilidæ), and Sun Birds (Nectarinidæ). In these birds it is clear that colour has had full sway, untramelled by any necessity for modification. Nothing is more striking than the mapping out of the surface of these birds into regions of colour, and these regions are always bounded by structural lines. Take, for instance, _Paradisea regia_. In this bird we find the following regions mapped in colour:-- Sternum brown. Clavicle yellow. Pelvis yellow. Band brown. Frontal bone black. Parietal bones green. Occiput yellow. A beautiful ruff emphasizes the pectoral muscles, and the tail appendages emphasize the share-like caudal vertebræ. If we turn to the other species of this genus, we find in _P. Papuana_ the claret breast suddenly change to green at the furculum; and similar changes take place in _P. speciosa_, while in _P. Wallacei_ and _Wilsoni_ this region is decorated with a wonderful apron of metallic green. The region of the furculum is equally well marked in the Toucans and Sun-birds.