In the course of
time they lose the teeth--an advantage in the distribution of the
weight of the body while flying--and develop horny beaks. In the
gradual shaping of the breast-bone and head, also, they
illustrate the evolution of the bird-form.
But the birds were meantime developing from a quite different
stock, and would replace the Pterosaurs at the first change in
the environment. There is ground for thinking that these flying
reptiles were warm-blooded like the birds. Their hollow bones
seem to point to the effective breathing of a warm-blooded
animal, and the great vitality they would need in flying points
toward the same conclusion. Their brain, too, approached that of
the bird, and was much superior to that of the other reptiles.
But they had no warm coats to retain their heat, no clavicle to
give strength to the wing machinery, and, especially in the later
period, they became very weak in the hind limbs (and therefore
weak or slow in starting their flight). The coming selection will
therefore dismiss them from the scene, with the Deinosaurs and
Ammonites, and retain the better organised bird as the lord of
the air.
There remain one or two groups of the Mesozoic reptiles which are
still represented in nature.
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