ALTHOUGH placed next to the Penguins, the members of the present order are of course quite markedly different from them, but, as suggested in the chapter on classification, this disposition illustrates one of the difficulties of a linear arrangement, and it does not follow that all placed contiguously are equally related. However, the Colymbiformes are also a group of ancient origin and likewise hold a somewhat isolated position. They were formerly associated with the Auks, and as a matter of fact they not only show affinities with them, but with several other groups, yet comparatively recent anatomical studies have demonstrated that they are, on the whole, abundantly distinct and well entitled to be ranked as a separate order. They are all markedly aquatic birds of medium or large size, with moderately long necks, long, mostly sharp-pointed bills, usually strong wings, and webbed or lobed toes. Perhaps the most marked anatomical features are the laterally flattened metatarsi, and the peculiar upward prolongation of the cnemial crest of the tibia.
The Colymbiformes are divided into two families, the Gavidae, or Loons, and the Podicipedidae, or Grebes. The first is distinguished by having the hind toe on a level with the other toes, which are united by a web, and by a tail of eighteen or twenty short, stiff feathers. In the Grebes the hind toe is raised above the level of the others, while the toes are provided with wide lateral lobes, and the tail usually so reduced as to be indistinguishable, though, as Pycraft has recently shown, it is never really absent.