for the longest pinions. In many reptiles, such as Crocodiles, there are
peculiar bones running across the abdomen beneath the skin, the
so-called "abdominal ribs," and it seems an eloquent detail to find
these represented in _Archaeopteryx_, the earliest known bird. No modern
bird shows any trace of them. [Illustration: SKELETON OF AN EXTINCT
FLIGHTLESS TOOTHED BIRD, HESPERORNIS

(_After Marsh._)

The bird was five or six feet high, something like a swimming ostrich,
with a very powerful leg but only a vestige of a wing. There were sharp
teeth in a groove. The modern divers come nearest to this ancient
type.]

[Illustration: SIX STAGES IN THE EVOLUTION OF THE HORSE, SHOWING GRADUAL
INCREASE IN SIZE

(_After Lull and Matthew._)

1. Four-toed horse, Eohippus, about one foot high. Lower Eocene, N.
America.

2. Another four-toed horse, Orohippus, a little over a foot high. Middle
Eocene, N. America.

3. Three-toed horse, Mesohippus, about the size of a sheep. Middle
Oligocene, N. America.

4. Three-toed horse, Merychippus, Miocene, N. America. Only one toe
reaches the ground on each foot, but the remains of two others are
prominent.

5. The first one-toed horse, Pliohippus, about forty inches high at the
shoulder. Pliocene, N. America.

6. The modern horse, running on the third digit of each foot.]

There is no warrant for supposing that the flying reptiles or
Pterodactyls gave rise to birds, for the two groups are on different
lines, and the structure of the wings is entirely different. Thus the
long-fingered Pterodactyl wing was a parachute wing, while the secret of
the bird's wing has its centre in the feathers. It is highly probable
that birds evolved from certain Dinosaurs which had become bipeds, and
it is possible that they were for a time swift runners that took "flying
jumps" along the ground. Thereafter, perhaps, came a period of arboreal
apprenticeship during which there was much gliding from tree to tree
before true flight was achieved. It is an interesting fact that the
problem of flight has been solved four times among animals--by insects,
by Pterodactyls, by birds, and by bats; and that the four solutions are
on entirely different lines.

In the _Cretaceous_ period the outstanding events included the waning of
giant reptiles, the modernising of the flowering plants, and the
multiplication of small mammals. Some of the Permian reptiles, such as
the dog-toothed Cynodonts, were extraordinaril