Another factor demonstrating the impossibility of the reptile-bird evolution scenario is the structure of avian lungs, which cannot be accounted for by evolution.
In land-dwelling creatures, air flow is bidirectional. Upon inhaling, the air travels through the passages in the lungs (bronchial tubes), ending in tiny air sacs (alveoli). The exchange of oxygen and carbon dioxide takes place here. Then, upon exhaling, this used air makes its way back and finds its way out of the lung by the same route.
In birds however, air is unidirectional. New air comes in one end, and the used air goes at the other end. By means of special air sacs all along the passages between them, air always flows in one direction through the avian lung. In this way, birds are able to take in air nonstop. This satisfies birds' high energy requirements. This highly specialized respiratory system is explained by Michael Denton in his book A Theory in Crisis:
In the case of birds, the major bronchi break down into tiny tubes which permeate the lung tissue. These so-called parabronchi eventually join up together again, forming a true circulatory system so that air flows in one direction through the lungs. ...[T]he structure of the lung in birds and the overall functioning of the respiratory system is quite unique. No lung in any other vertebrate species is known which in any way approaches the avian system. Moreover, it is identical in all essential details in birds as diverse as humming birds, ostriches and hawks.111
The important thing is that the reptile lung, with its bidirectional air flow, could not have evolved into the bird lung with its unidirectional flow, because it is not possible for there to have been an intermediate model between them. In order for a creature to live, it has to keep breathing, and a reversal of the structure of its lungs would inevitably end in death. According to evolution, this change must happen gradually over millions of years, whereas a creature whose lungs do not work will die within a few minutes.
Michael Denton states that it is impossible to give an evolutionary account of the avian lung:
Just how such an utterly different respiratory system could have evolved gradually from the standard vertebrate design is fantastically difficult to envisage, especially bearing in mind that the maintenance of respiratory function is absolutely vital to the life of an organism to the extent that the slightest malfunction leads to death within minutes. Just as the feather cannot function as an organ of flight until the hooks and barbules are coadapted to fit together perfectly, so the avian lung cannot function as an organ of respiration until the parabronchi system which permeates it and the air sac system which guarantees the parabronchi their air supply are both highly developed and able to function together in a perfectly integrated manner.112
Parabronchial tubes, which enable air to circulate in the right direction in birds' lungs. Each of these tubes is just 0.5 mm. in diameter.
In brief, the passage from a terrestrial lung to an avian lung is impossible, because an intermediate form would serve no purpose.
Another point that needs to be mentioned here is that reptiles have a diaphragm-type respiratory system, whereas birds have an abdominal air sac system instead of a diaphragm. These different structures also make any evolution between the two lung types impossible, as John Ruben, an acknowledged authority in the field of respiratory physiology, observes in the following passage:
The earliest stages in the derivation of the avian abdominal air sac system from a diaphragm-ventilating ancestor would have necessitated selection for a diaphragmatic hernia in taxa transitional between theropods and birds. Such a debilitating condition would have immediately compromised the entire pulmonary ventilatory apparatus and seems unlikely to have been of any selective advantage.113
Another interesting structural feature of the avian lung which defies evolution is the fact that it is never empty of air, and thus never in danger of collapse. Michael Denton explains the position:
Just how such a different respiratory system could have evolved gradually from the standard vertebrate design without some sort of direction is, again, very difficult to envisage, especially bearing in mind that the maintenance of respiratory function is absolutely vital to the life of the organism. Moreover, the unique function and form of the avian lung necessitates a number of additional unique adaptations during avian development. As H. R. Dunker, one of the world's authorities in this field, explains, because first, the avian lung is fixed rigidly to the body wall and cannot therefore expand in volume and, second, because of the small diameter of the lung capillaries and the resulting high surface tension of any liquid within them, the avian lung cannot be inflated out of a collapsed state as happens in all other vertebrates after birth. The air capillaries are never collapsed as are the alveoli of other vertebrate species; rather, as they grow into the lung tissue, the parabronchi are from the beginning open tubes filled with either air or fluid.114
In other words, the passages in birds' lungs are so narrow that the air sacs inside their lungs cannot fill with air and empty again, as with land-dwelling creatures.
If a bird lung ever completely deflated, the bird would never be able to re-inflate it, or would at the very least have great difficulty in doing so. For this reason, the air sacs situated all over the lung enable a constant passage of air to pass through, thus protecting the lungs from deflating.
Of course this system, which is completely different from the lungs of reptiles and other vertebrates, and is based on the most sensitive equilibrium, cannot have come about with unconscious mutations, stage by stage, as evolution maintains. This is how Denton describes this structure of the avian lung, which again invalidates Darwinism:
The avian lung brings us very close to answering Darwin's challenge: "If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down."115
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