. The evolution of the earth and its inhabitants; a series delivered before the Yale chapter of the Sigma xi during the academic year 1916-1917. FIG. 24.—Skull of cynodont reptile, Nythosaurus!ar<vatus. Note the mammal-like tooth differ-entiation, but complex reptilian jaw. AfterBroom, from the Pirsson-Schuchert Text-book of Geology, published by John Wiley& Sons, Inc. Origin of birds. Birds, on the other hand, came fromanother, unrelated reptilian stock, that which also producedthe dinosaurs. True flight, such as that which the birds de-veloped, has been thrice evolved among the vertebrate
. The evolution of the earth and its inhabitants; a series delivered before the Yale chapter of the Sigma xi during the academic year 1916-1917. FIG. 24.—Skull of cynodont reptile, Nythosaurus!ar<vatus. Note the mammal-like tooth differ-entiation, but complex reptilian jaw. AfterBroom, from the Pirsson-Schuchert Text-book of Geology, published by John Wiley& Sons, Inc. Origin of birds. Birds, on the other hand, came fromanother, unrelated reptilian stock, that which also producedthe dinosaurs. True flight, such as that which the birds de-veloped, has been thrice evolved among the vertebrates andonce among the invertebrates. In the two other vertebrategroups, however, the flying mechanism involves not only thefore limbs, as in the bird, but also the hinder pair. With thebirds, moreover, the hind limbs, as in dinosaurs, show a dis-. FIG. 23.—Diagram showing the relation of the extra-embryonal membranes. A, bird or reptile, withfunctional yolk-sac and respiratory allantois; B,mammal with functionless yolk-sac and with theallantois converted into an umbilical cord andplacenta. From Wilders History of the HumanBody, published by Henry Holt & Company. AND ITS INHABITANTS 129 tinct adaptation of another sort, that for swift movement,hence the bird-like hind limbs of the dinosaur—or perhaps theconverse of this statement would be more nearly true. Swiftmovement, which in this instance implies bipedality, evidentlypreceded flight, and its impression on the bird was so greatthat it has never been entirely relinquished despite the attain-ment of the higher faculty. Whether birds passed through anarboreal condition or whether they arose direct from cursorialtypes is unknown. It hardly seems probable, however, thatmore than one evolution occurred. We may conceive of aproavian as being at least partially arboreal and
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