"The evolution of birds is far more complex than the above discussion implies. In addition to the problem of the origin of the feather and flight, birds possess other unique adaptations which also seem to defy plausible evolutionary explanations. One such adaptation is the avian lung and respiratory system.
In all other vertebrates the air is drawn into the lungs through a system of branching tubes which finally terminate in tiny air sacs, or alveoli, so that during respiration the air is moved in and out through the same passage.
In the case of birds, however, the major bronchi break down into tiny tubes which permeate the lung tissue (see Figure 9.2). These so- called parabronchi eventually join up together again, forming a true circulatory system so that air flows in one direction through the lungs.
This unidirectional flow of air is maintained during both inspiration and expiration by a complex system of interconnected air sacs in the bird’s body which expand and contract in such a way so as to ensure a continuous delivery of air through the parabronchi. The existence of this air sac system in turn has necessitated a highly specialized and unique division of the body cavity of the bird into several compressible compartments. Although air sacs occur in certain reptilian groups, the 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.
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.
Moreover, the unique function and form of the avian lung necessitates a number of additional unique adaptations during avian development.
In attempting to explain how such an intricate and highly specialized system of correlated adaptations could have been achieved gradually through perfectly functional intermediates, one is faced with the problem of the feather magnified a thousand times.
The suspicion inevitably arises that perhaps no functional intermediate exists between the dead-end and continuous through-put types of lung. The fact that the design of the avian respiratory system is essentially invariant in ALL birds merely increases one’s suspicion that no fundamental variation of the system is compatible with the preservation of respiratory function. One is irresistibly reminded of Cuvier’s view that the great divisions of nature are grounded in necessity and that intermediates cannot exist because such forms are incoherent and non functional.”
(Evolution: A Theory In Crisis, Michael Denton :210-212)