No Class of living animals is more homogeneous than Birds; and well-preserved remains prove that, at least as far back in time as the Upper Oolites, birds were clothed with feathers of essentially the same mode of growth and appearance as the feathers of living birds. There may, therefore, be no ground for assuming that the covering was ever different, though some regions of the skin are free from feathers. Yet the variations from fine under-down to the scale-like feathers on the wings of a Penguin, or the great feathers in the wings of birds of flight, or the double quill of the Ostrich group, are calculated to yield dissimilar impressions in a fossil state, even if the fine down would be preserved in any stratum. VARIATION IN THE BONES OF BIRDS Osteologically there is less variety in the skeleton of birds than in other great groups of animals. The existing representatives do not exhaust its capability for modification. The few specimens of birds hitherto found in the Secondary strata have rudely removed many differences in the bones which separated living birds from reptiles; so that if only the older fossil birds were known, and the Tertiary and living birds had not existed, a bird might have been defined as an animal having its jaw armed with teeth, instead of devoid of teeth; with vertebræ cupped at both ends, instead of with a saddle-shaped articulation which in front is concave from side from side, and convex from above downwards; in which the bones of the hand are separate, so that three digits terminating in claws can be applied to the ground, instead of the metacarpal bones being united in a solid mass with clawless digits; and in which the tail is elongated like the tail of a lizard. Yet the limits to variation are not to be formulated till Nature has exhausted all her resources in efforts to preserve organic types by adapting them to changed circumstances. Birds may be regarded theoretically as equally capable with mammals of parting with almost every distinctive structure in the skeleton by which it is best known. Even the living frigate bird blends the early joints of the backbone into a compact mass like a sacrum. The Penguin has a cup-and-ball articulation in the early dorsal vertebræ, with the ball in front. And the genus Cypselus has the upper arm bone almost as broad as long, unlike the bird type. Such examples prove that we are apt to accept the predominant structures in an animal type as though they were universal, and forget that inferences based, like those of early investigators, on limited materials may be re-examined with advantage. VARIATION IN THE BONES OF REPTILES