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The association of the Families usually arranged in one group, under the above title, or that of Zygodactyli or yoke-footed birds, is by most naturalists felt to be unsatisfactory. Unlike in food, in form, in habits, and economy, the single character which they have in common, is that their four toes are arranged in two pairs, the outer toe being turned backward more or less permanently, like the thumb, so that these are opposible to the middle and inner toes, which point in the opposite direction. From this structure results a more efficient power of grasping, or of clinging to perpendicular or reversed surfaces, associated with climbing habits in the principal Families, as those of the Parrots and the Woodpeckers.

In the other Families, however, those of the Toucans and the Cuckoos, this disposition of the toes is not accompanied with the power of climbing, properly so called; though the latter, and perhaps the former, do certainly move about the branches of trees, in a manner diverse from that employed by the true perching or Passerine birds. At the same time it must be borne in mind that the faculty of climbing, even if common to the whole of this Order, is by no means peculiar to it; as the Creepers and Nuthatches, whose toes are arranged on the Passerine type, can climb and