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work into the substance of coral, and Teredines are found to have rasped their way through the masses of drift wood in the Portland oolite, the Woburn sand, the Chalk and London clay.
Movements in air are accomplished by animals of the articulated and vertebrated types with a considerable variety of organization, curiously engrafted on the original structure by modification of some of its parts. If we consider the mechanism for flight in its completeness, the general idea appears to be the employment of two anterior limbs for rowing through the air by an expanded elastic wing, and employing some retral instrument for steerage. Thus the general arrangement is in a considerable degree the inverse of that employed for swimming. The wing or air-fin is in like manner distinct in principle of construction from the paddle or water-fin. Its anterior edge is always strongly fortified-by bones in Birds, by strong tracheal tubes in Insects-the surface is made flexible by feathers in Birds, by clear or scaly membranes in Insects. Thus a yielding blade is presented to the air, and this yielding is so managed in Birds as to be nothing anteriorly but great retrally, very little on the stroke, very great on the return. This is managed (1) by the tile-like arrangement of the feathers as a whole; for thus they are strongly compacted in one direction, and feeble in
the other; (2) by the bending of the jointed wing to extend or reduce the surface, and increase or diminish the flexibility; and (3) by the construction of each effective wing-feather. For each of these is so made, both in regard to the axis of the plume, and in regard to the smaller elements of the feather, as to be strong in the direction in which the whole wing-arrangement is strong, and yielding in the direction in which the whole wing is yielding.
This remarkable concurrence of the individual strength of the feather in all its parts with the mechanical conditions of the problem is secured by the general shape and also by a change of the substance of the feather on the opposite faces; the upper convex face of the wing-feather having the compact quill-sheath extended along it to the very extremity and giving origin to the plume, while below it is a thick mass of cellular substance, not so covered, in which extension and contraction take place advantageously. This is exactly the arrangement indicated by the experiments of engineers, and the theories of mechanicians, for the employment of the least weight of such elastic materials in the production of the required result1; but no human hands could make an apparatus embodying so perfectly the abstract
1 Hodgkinson and Fairbairn, in Reports of the British Association. Barlow, On Strength of Materials.
truths of mechanical science, nor could the human mind with the materials given have predicted by any theory the arrangement which is found to be so complete. Into the other wonders of the feather, its hollow quill basis, its implantation, the relation of the directions of its axis and plane to the axes and planes of the plumose elements I cannot now enter, though by this means the impression that the whole is an ingenious and profound piece of mechanism would surely be strengthened.
The scheme of feather structure and arrangement is altered in the tails of Birds to suit the very different mechanical purposes of that instrumentaltered in the construction of the individual feathers -in the direction of their planes-in the resultant of their strength. Hence the resemblance of the steering-tails of the swift Falconidæ, and Hirundines, and Sternidæ in contrast with the stiff prop of the Picidæ, and the almost extinction of the apparatus and suppression of the function in the acuminated tails of the Diving-birds. Every feather is altered when the work is different.
Among Insects the Lepidoptera, with their expanded wings, shew the same principle, differently carried out; what in human contrivances is called 'feathering the oar' being largely employed in nature; and in Coleoptera we may cite the curious
doubling and cross-folding of the membranous wings under their waterproof horny elytra, as a singular and expressive example of a purely mechanical operation, performed on an organ for its conservation, more ingenious than the putting-up of a fan in its varnished case, or a parapluie into its oiled sheath.
The same mechanical principles may be traced in the wings of the Bat, carried out in practice in such a manner as to suit the Mammalian type of bones and dermal covering. The fluttering collapse of the soft skin of the Vespertilionidæ is however a much lower order of movement in air than the rapid sweep of the elastic feather-wing of the bird; nor need we suppose for the flying Lizards of the Mesozoic ages a very rapid or very sustained flight, notwithstanding the hollowness of their elongated digital bones, the carinated sternum, and the devotion of a large part of the muscular energy to the anterior limbs. But there is no reason whatever for comparing the Pterodactyle with such parachute-bearing Mammals as Petaurus, and Pteromys, and still less with the Lizard called Draco volans. The membranes extended between the limbs of the former creatures, and beyond the ribs of the latter, serve only to retard their fall, and thus to allow of their making longer leaps, especially among trees, just as the flying-fish makes long leaps by the sustaining power of its expanded pec
toral fins. Pterodactylus, by its large and pointed wings, retractile neck and snapping long-toothed jaws, seems to realize all that can be supposed of a reptile accustomed to flap the air rather heavily not far above shallow waters, and occasionally to snatch from them fishes swimming near enough to the surface to come within reach.
Life in Trees presents us with a considerable variety of contrivances for holding to the surface, grasping the branches, or making incisions into the bark and wood. To say nothing of the sucker feet of Dipterous Insects, and the Gecko Lizard, we may remark on the prevalent idea of climbing and holding on by opposable fingers which appear among the Reptilia in Chamæleon, among the Birds in the Parrot and Woodpecker, and among the Mammalia in the whole race of the Quadrumana. Perhaps the prehensile tail of the Platyrhine Monkeys, and the suspensorial claws of the Sloth, may be quoted among the singular provisions of animals of the New World. It is even more curious to notice with respect to some of these climbing animals, the other adjustments which complete their equipment. Thus the feeding of the Woodpecker is provided for not only by its scansorial foot, its supporting tail, and its perforating beak, which makes the forests ring around. We have further to notice the singular