A common objection posed to the account I have sketched of how natural selection gives rise to otherwise improbable features, is that some postulated transitions, for example from a leg to a wing, cannot be adaptive. The answer to this kind of objection is well known to evolutionists. For example, there are rodents, primates, and other living animals that exhibit modified legs used for both running and gliding. The fossil record famously includes the reptile Archaeopterix and many other intermediates showing limbs incipiently transformed into wings endowed with feathers.

One challenging transition involves the bones that make up the lower jaw of reptiles but have evolved into bones now found in the mammalian ear. What possible function could a bone have, either in the mandible or in the ear, during the intermediate stages? However, two transitional forms of therapsids (mammal-like reptiles) are known from the fossil record with a double jaw joint—one joint consisting of the bones that persist in the mammalian jaw, the other composed of the quadrate and articular bones, which eventually became the hammer and anvil of the mammalian ear.