A few other and more familiar examples may complete this discussion.  Among Climbing Birds, for instance, which are held together as a more comprehensive group by the structure of their feet and by other anatomical features, there are two Families so widely different in their form that they may well serve as examples of this principle.  The Woodpeckers (Picidae) and the Parrots (Psittacidae), once considered as two Genera only, have both been subdivided, in consequence of a more intimate knowledge of their generic characters, into a large number of Genera; but all the Genera of Woodpeckers and all the Genera of the Parrots are still held together by their form as Families, corresponding as such to the two old Genera of Picus and Psittacus.  They are now known as the Families of Woodpeckers and Parrots; and though each group includes a number of Genera combined upon a variety of details in the finish of special parts of the structure, such as the number of toes, the peculiarities of the bill, etc., it is impossible to overlook the peculiar form which is characteristic of each.  No one who is familiar with the outline of the Parrot will fail to recognize any member of that Family by a general form which is equally common to the diminutive Nonpareil, the gorgeous Ara, and the high-crested Cockatoo.  Neither will any one, who has ever observed the small head, the straight bill, the flat back, and stiff tail of the Woodpecker, hesitate to identify the family form in any of the numerous Genera into which this group is now divided.  The family characters are even more invariable than the generic ones; for there are Woodpeckers which, instead of the four toes, two turning forward and two backward, which form an essential generic character, have three toes only, while the family form is always maintained, whatever variations there may be in the characters of the more limited groups it includes.

The Turtles and Terrapins form another good illustration of family characters.  They constitute together a natural Order, but are distinguished from each other as two Families very distinct in general form and outline.  Among Fishes I may mention the Family of Pickerels, with their flat, long snout, and slender, almost cylindrical body, as contrasted with the plump, compressed body and tapering tail of the Trout Family.  Or compare, among Insects, the Hawk-Moths with the Diurnal Butterfly, or with the so-called Miller,—­or, among Crustacea, the common Crab with the Sea-Spider, or the Lobsters with the Shrimps,—­or, among Worms, the Leeches with the Earth-Worms,—­or, among Mollusks, the Squids with the Cuttle-Fishes, or the Snails with the Slugs, or the Periwinkles with the Limpets and Conchs, or the Clam with the so-called Venus, or the Oyster with the Mother-of-Pearl shell,—­everywhere, throughout the Animal Kingdom, difference of form points at difference of Families.