And here s an article from everyones favorite Biologist, historian, linguist..
by Frank R. Zindler
The Probing Mind, April, 1986
Creationists are very fond of the argument that evolution literally won't fly - especially in the case of birds - because "half a wing is worse than no wing at all." The implication is that in the evolution of arms into wings, intermediate stages would be produced which would be usable neither as arms nor wings. A similar argument is leveled against the idea that the human eye evolved: half an eye would be useless, they claim.
When creationist big-wigs use such arguments, they are al-most certainly intending to deceive, for they know perfectly well that vast amounts of material have been written developing step-by-step scenarios for the evolution of both eyes and wings - scenarios in which every stage in the evolution of the eye is useful, and every step-in the evolution of wings confers a survival advantage upon the so-called proavian, the bird-to-be.
The Evolution Of Flight
Although some very famous ornithologists have thought otherwise, I am convinced that the majority opinion is correct: bird flight began in the trees, not on the ground. Having evolved from small, two-legged dinosaurs, the proavians were arboreal bipeds. Proavis used its "hands" to climb‚ into trees in much the same way that young hoatzins - primitive South American birds - today still use their finger-remnants to scuttle back up to their nests if they fall or jump out of them. But the major mode of proavian locomotion while in the tree, I believe, was bipedal hopping.
Hopping short distances from branch to branch presented few problems for Proavis. (Proavis, keep in mind, was a precursor of birds, not a prehistoric rental car!) But jumping greater distances, especially from higher to lower branches, created a serious problem. Upon landing with its feet on a target branch, momentum would have tended to rotate the animal forward on the branch - making it end up hanging from the branch upside-down! The situation would be even worse if the branch were flexible instead of rigid. Suitable braking, by flailing the arms, would have helped to prevent this - as readers can easily demonstrate for themselves if they set up two chairs several feet apart and then try to jump from one to the other, without flailing the arms, and without falling off or tipping over the target chair. As the distance between chairs increases, the experimenter's arm reflexes will quickly demonstrate how important arm-flailing must have been to the bipedal branch-hopping proavians.
The lengthening and fraying of reptilian scales to form feathers would have conferred a great selective advantage upon such creatures as I have envisioned. (Many birds still retain reptilian scales on their legs, remember, and even the barnyard hen occasionally sports structures which are part-scale, part-feather.) Body feathers may very well have existed long before flight evolved as a means of thermal insulation allowing the animal to maintain the high body temperatures needed for active life in trees. Longer feathers, such as wing and tail feathers (often the first non-down feathers to develop in the young of songbirds) first evolved not for flight, but as a means of improving aerial braking and balancing capability as Proavis hopped and parachuted from branch to branch.
The larger surface area provided by the feathered limb would allow for more effective "air-braking," and would make possible later reduction of the inertial mass of the arms and other parts of the body - a handy thing still later when active flight developed. Proavis probably behaved a lot like the previously mentioned hoatzin which, even as an adult bird capable of clumsy flight, seems to spend more time flapping its wings to keep from falling off branches than it does in active flight!
Among the many combinations and sequences of muscle and joint movements involved in air-braking were those needed for the power-stroke and other flight movements. It would only require a bit of selective shaping to make the power-stroke anatomically less stressful and more powerful. The transition from flapping, parachuting and braking, to active, flapping flight would have been very gradual and easy, without any maladaptive intermediate stages. The oldest avian fossil, Archaeopteryx, by the way, is a marvelous connecting link between two-legged dinosaurs and modern birds. It had a long, lizard-like tail, teeth, clawed wing-digits still usable for climbing, and many other reptilian skeletal features too numerous to mention here. In fact, the only thing bird-like about it was its feathers. Of course, the creationists deny this. After decades of insisting that Archaeopteryx was a perfectly complete and genuine bird, they now are echoing the off-the-wall claims of the astronomer Fred Hoyle that all the fossils of this creature are hoaxes. No matter which museum the fossils are in, we are to believe, the curators have chiseled feather imprints into the limestone!
Flight has evolved independently a number of times in the history of the animal kingdom: among the bats (mammals), among the birds, among the pterosaur reptiles (pterodactyls and their cousins), and among the insects. In each case, natural selection has used different means to achieve the same end.
In the three vertebrate cases mentioned, wings have evolved by alteration of the anterior paired appendages (the "arms"), with differing emphasis on which parts of the appendage will produce the major part of the wing. In the case of birds, almost all the parts of the arm figure prominently in the structure of the wing. In the case of bats, the bones of the hand and fingers account for the major flight surface. (A bat flies by flapping its hands.) In the pterosaurs, a single finger became extremely long and served as a strut along which a sail-like wing of leathery skin was stretched.
In the insects, however, there was no preexisting locomotor appendage which could be modified for flight, and so wings originated in a manner quite differently than in the vertebrates. The protowings of insects apparently had nothing whatsoever to do with locomotion, let alone flight. Rather, they began as flat, horizontal outgrowths from the upper surface of the middle section of the body (the thorax). What was the function of these plate-like structures? It is now all but certain that these structures were primitive solar-energy collectors -- although thin platelike body projections could also have helped preflight aquatic insects wind-sail across the surfaces of bodies of water.
Insects are "cold-blooded" animals, and tend to be lethargic or inactive when the temperature of their environment is low. If, however, a way be found to increase the amount of solar energy absorbed, it becomes possible to warm the blood and, by pumping warmed blood through the body-core and nervous system, to remain active at temperatures otherwise too cold to allow insect activity. The simplest way to increase the amount of solar energy absorbed is to increase the body surface area, while increasing body volume as little as possible. Any flat, horizontal outgrowth from the upper surface of the body will suffice, and a variety of such structures are known in both living and fossil insects. In the case of those outgrowths which were destined to become wings, the structures were located on the three thoracic segments of the body, the same segments on which the walking legs are located.
Any mutations increasing the size of these solar-collectors would tend to increase the probability of survival, by increasing the amount of blood-warming energy absorbed, and we would expect to see these structures increase rapidly in size. A further advantage would be gained by animals with mutations causing superficial thoracic muscles to become attached to the base of these solar panels, allowing them to be bent or arrayed at various angles to the sun and, thus, to modulate the amount of energy absorbed. Once such moveable solar panels had increased to a certain size, they would begin to serve as protowings. At first the protowings would be used only for gliding flight and to assist in wind-dispersal of the animals into new environments. But natural selection would rapidly increase muscular control and motility of the structures, and full-fledged wings would result.
That this is indeed the way that insect wings evolved is supported not only by computer-modeling studies, but also by studies of both living and fossil insects. Many living insects still use their wings as solar-energy collectors, pumping blood into the veins of the wings (often by means of special auxiliary hearts) and returning the heated blood straight from the wings to the brain. Even though wings have added flight to the behavioral repertoire of insects, wings still retain their primitive thermo-regulatory function.
A number of extremely interesting fossil insects are known which, for a long time, have been thought to display a pair of degenerate wings on the first thoracic segment, just in front of the two pairs of full-sized wings [Fig. 1]. It now seems more likely that these are not degenerate wings, but rather solar-panels which never expanded into full-fledged wings - probably because three pairs of wings create more aerodynamic problems than they solve. In at least some of these fossil forms, distinct veins are visible in these would-be wings ("paranota"), and it is highly probable that they served a blood-warming function. Thus, in the case of insects as well as birds, half a wing was better than no wing at all. But in contrast to the case of birds, where the protowing still served a locomotor function, the protowing of insects served a function only remotely related to locomotion
The Evolution of Eyes
Probably no organ of the body has excited more wonder among scientists and laymen alike than has the eye. In its complexity and exquisiteness of function, the human eye almost begs to be described as a miracle. But like other "miracles" of nature, the vertebrate eye has yielded up its secrets to science. Unlike the case of bird evolution, however, we have no fossils to guide us in our study of the evolution of the vertebrate eye, partly because it evolved very early in geologic history, and partly because soft organs do not usually fossilize. Fortunately, we have evidence far better than fossils to show us how eyes may have developed. Not only can we learn a lot about the stages of ocular evolution from the study of how eyes develop in the vertebrate embryo, organisms still living exhibit photoreceptor organs smoothly spanning the structural spectrum from the "eye-spots" of single-celled organisms to eyes even more complex than the human eye.
Many single-celled organisms, including bacteria and flagellates such as Euglena and Chlamydomonas [see Fig. 2-A], contain tiny granules of pigment which, after absorbing light, generate an electrical signal which can alter the behavior of the cell (usually by stimulating or inhibiting the beating of the whip-like flagellum). In a very real sense, the entire organism is an eye! At the risk of getting ahead of our story, we may note that the light-absorbing parts of the rods and cones in our own eyes are modified cilia (small flagella). In certain colonial forms of flagellates, such as Volvox [Fig. 2-B], although each cell in the colony is photosensitive, the colony as a whole is guided in its swimming, because the "eyes" on the sunny side of the colony move their flagella at a rate different from that on the shady side. Volvox is, as it were, a colony of eyes.
In certain flatworms, the most primitive worms known, only some of the cells of the body retain photopigments - usually cells on the upper (dorsal) surface of the body. In some cases, the entire dorsal surface is photosensitive. This is even true of "higher" worms such as the earthworm. But in many other worms, the photosensitive cells are bunched together into eye-like spots, as in Planaria, the cross-eyed worm beloved of all freshman biology students [Fig. 2-C]. The planarian eye cannot form images, since it has no lens to focus light patterns on the "retina" - the patch of photosensitive cells. Nevertheless, these worms make do with retinas able only to determine the direction from which light is coming. Theoretically, they should be able to determine the direction of shadows moving across their heads, but it is uncertain if their nervous system is complex enough to accomplish this feat of data processing.
If the eyespots of an animal are grooved or pitted, the photoreceptive surface area can increase without any overall increase in the size of the spot, thus increasing the photosensitivity of the spot. In addition, photoreceptors located at the bottom of the groove or pit are protected from abrasion - an ever-present hazard for cells located on the body surface. Eyes at this level of development are found in a number of mollusks, the limpet Patella being an example. If the photoreceptor cells are surrounded by cylinders of light-insulating pigment cells, [Fig. 2-D], cells located at different positions in the pit will be selectively sensitive to light coming from different directions. At this point, it is but a short step to the level of a pin-hole camera eye, such as we find in the cephalopod mollusk Nautilus, a distant cousin of the octopus [Fig. 2-E].
Whereas the previous photoreceptor organs were capable only of detecting light and shadow - and perhaps movements - with an eye such as that of Nautilus it is possible to project an image on the photoreceptor cells (the "retina") and, depending upon the data-processing capabilities of the nervous system connected to the retina, it is possible to analyze images and "recognize" objects in the environment. The most elaborate eyes known do no more than this, but many improvements can be made in the efficiency and precision with which this is done.
Although the pin-hole camera eye can form images, the quality of the image is poor, and there is always the danger that sand or parasites might get into the eye through the open hole, or that the hollow structure might tend to collapse. By filling the eye cavity with a clear, jelly-like secretion, not only can foreign objects be kept away from the retina, the danger of collapse can be avoided. Moreover, if the secretion has an index of refraction different from that of water (or air, if aerial vision is involved), it will be able to serve as a primitive lens and will improve the focus of the pin-hole image. An example of this stage of evolution is the eye of the abalone Haliotis [Fig. 2-F].
Once a lens-like object is present in the eye, it is no longer necessary to keep the pin-hole open, provided that the cells covering the hole be transparent. Hardening and contraction of the lens material into a spherical or ellipsoidal shape, as in the case of the snail Helix [Fig. 2-G], will further improve image resolution. By suspending the lens at variable distances from the retina, by developing ways of distorting the shape of the lens, by adding an iris diaphragm allowing for adjustment of the amount of light passing through the lens, and by hardening the layer of cells covering the eye into a light-focusing cornea, natural selection has produced the eye of the cuttlefish Sepia [Fig. 2-H], an eye as complex as that of humans.
As an organ developed via the opportunistic twists and turns of evolutionary processes, the human eye is explainable. As an organ designed and created by an infinitely wise deity, the human eye is inexcusable. For unlike the invertebrate eyes just studied, the human eye is constructed upon the foundation of an almost incredible error: the retina has been put together back-wards! Unlike the retinas of octopuses and squids, in which the light-gathering cells are aimed forward, toward the source of incoming light, the photoreceptor cells (the so-called rods and cones) of the human retina are aimed backward, away from the light source. Worse yet, the nerve fibers which must carry signals from the retina to the brain must pass in front of the receptor cells, partially impeding the penetration of light to the receptors. Only a blasphemer would attribute such a situation to divine design!
Although the human eye would be a scandal if it were the result of divine deliberation, a plausible evolutionary explanation of its absurd construction can be obtained quite easily - even though we can make little use of paleontology (because eyes, like all soft tissues, rarely fossilize) or comparative anatomy (since all vertebrate eyes, from fish to philosopher, are of nearly equal complexity).
It appears that the vertebrate eye evolved very rapidly, at a very remote period of geological history, before vertebrates had developed fossilizable hard parts. The only clues we have with which to infer the stages of evolution of the vertebrate eye come from the study of its embryology. It is a well-known (though sometimes exaggerated) principle in biology that organisms relive their evolutionary history in the course of their embryological development. Although the embryological recapitulation of evolu-tionary history occurs only in a very general way, a plausible, coherent evolutionary history of the human eye can be inferred from its embryological development.
At a very early stage in its development, the human embryo is constructed like a primitive worm [Fig. 3-A]. At this stage, conceptually it is little more than a tube within a tube, with the inner tube (endoderm) destined to become the digestive tract, the outer tube (ectoderm) destined to become the skin and nervous system, and the cells filling in the space between the tubes (the mesoderm) destined to become tissues such as blood, muscle, and bone. At this stage no nervous system as such has yet formed. However, the ectoderm running down the middle of the back (the neurectoderm) will become grooved [Fig. 3-B] and will sink down into the embryo to become a hollow neural tube [Fig. 3-C] - the structure from which the brain and spinal cord will develop as a result of thickening of the walls of the tube.
In the head region, the neural tube becomes greatly inflated, and two balloon-like out-pocketings, the optic vesicles, develop [Fig. 3-D]. As the optic vesicles grow toward the body surface, they become dimpled to form a cup-within-a-cup structure [Fig. 3-E], the outer layer of which will become the retina. At about the same time, the ectoderm (skin) immediately above the optic vesicle becomes dimpled, and the cells comprising the dimple become pinched off to form the lens of the eye [Fig. 3-F]. The cells remaining on the surface become the cornea. Other outgrowths of tissue (not shown in Fig. 3) develop to support the lens and to form an iris and superficial structures such as the eyelids.
In the course of all this development, cells which in the early embryo were oriented facing upward and outward [Fig. 3-A, cells with arrows in them] come to lie in the retina facing backward and inward [Fig. 3-F, arrow-marked cells]. In this way, the backwardly constructed human retina comes to be.
What can we infer from all this concerning the evolutionary history of the human eye?
In a nutshell, it appears that our distant ancestors were tiny, worm-like aquatic creatures which possessed photosensitive ciliated or flagellated cells down the middle of the upper surface of their bodies. As these worms evolved a central nervous system based upon a neural-tube structure, the photoreceptor cells came to lie in the lining of the cavity of the neural tube. Since the animals were very small and transparent, light could reach the receptor cells in the neural tube almost as easily as if they were still on the body surface. The primitive chordate known as Amphioxus - a small marine animal looking like a cross between a worm and an eyeless fish - still possesses such photo-sensitive cells in the lining of its neural tube.
As our ancestors grew bigger and more opaque (size is important if you want to rule the world!), it became necessary for the receptor cells to move away from the neural tube, in order to stay near the transparent surface of the body, all the while maintaining communication with the central nervous system. The optic vesicles which developed for this purpose, were destined to become the retina and associated structures, and the stalks connecting the optic vesicles to the neural tube became the optic nerves connecting the eyes to the brain.
It is likely that structural changes in the skin (ectoderm) overlying the optic vesicles occurred almost as soon as the optic vesicles had come to lie close to the body surface. Not only would the overlying skin have become especially transparent (in order not to shade the photoreceptors), changes in its refractive properties must have occurred very early also. Depending upon the surface curvature of this skin and its cross-sectional structure, a proto-lens was formed - at first capable only of increasing the light-gathering capacity of the eye, but ultimately capable of helping the lens to focus the light into a patterned image on the retina. With the development of the corneal portion of this ectoderm, the focusing capability of the eye would be greatly increased and would allow for aerial vision when, millions of years later, our ancestors left the waters which had spawned them and began their conquest of the land.
Neither wings nor eyes are miracles. Both organs betray the opportunistic nature of the evolutionary process. Just as there is more than one way to make a wing, so too there are many ways of making eyes. All this is intelligible if these structures are the result of natural selection acting upon randomly occurring mutations. But to say that these structures are the result of intelligent design is to imply that the designer is either whimsical or perverse - or both. It is difficult to understand how anyone could worship a deity more interested in playing ring-around-the-retina than in preventing visual defects and blindness in the creatures it has created.
In the evolutionary scenarios presented above, each step along the way is viable and, moreover, confers a selective advantage greater than that of the previous step. In no case does it require a leap of faith to expect that random mutation plus natural selection could transform any particular step to the following one. From the primordial granules of photosensitive pigment found in the most humble microbes of the sea to the eyes which peer through telescopes and focus their world in both space and time, an unbroken chain of natural causes and material effects is becoming ever more clear. At no time have supernatural forces intervened to guide the process, nor did they need to.
The great beauty of Darwin's theory of evolution by means of natural selection is that it has plucked the "Argument from Design" from the bag of tricks available to religious apologists who try to "prove" the existence of their various gods. We must lament, however, that it is taking an inordinate amount of time for the creationists to discover that their quivers are not filled with arrows capable of laying low the arguments of unbelievers -- but rather contain only the hot air that is the standard issue for religious apologists.