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Dinosaurs are members of the reptile family. When birds and reptiles are examined it can be seen that their physiologies are very different. First and foremost, birds are warm-blooded and reptiles cold-blooded. The cold-blooded reptile metabolism works slowly. Birds, on the other hand, consume a great deal of energy in a tiring activity such as flying. Their metabolisms are much faster than those of reptiles. Birds have to carry oxygen to their cells very quickly, which is why they are equipped with a special respiratory system.
Air travels in only one direction in their lungs, thus not delaying the organism's supply of oxygen. In reptiles, on the other hand, the air taken into the body leaves it by the same channels. Unidirectional flow is found only in the bird lung, and is a unique design. It is impossible for such a complex structure to have come about in stages. That is because this unidirectional flow system and the lung itself need to exist in perfect form at all times in order for the creature to survive. Michael Denton, a biologist known for his criticisms of Darwinism, has this to say on the subject:
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.4

References
- Michael J. Denton, Nature's Destiny, Free Press, New York, 1998, p. 361. ↩
