eans are smallest in
breadth, (because greatest in depth), the continents are united by
land-bridges such as those we have now uniting North and South America."

"And Alaska and Asia?" suggested Ted.

"Practically, yes. And probably, at one time, South America and
Australia. These land-bridges changed the direction of the ocean
streams. You know in the age of reptiles there was nothing to divide the
Atlantic from the Pacific. Added to that, the high mountain ranges took
the moisture out of the winds from the oceans, as the Rockies now do the
Pacific trade winds, so that by the time they reach Nevada there is no
moisture left in them to form clouds and fall in rain, and we have desert.

"Of course the animals that lived on the earth in its flatter, more
temperate stage now have to adapt themselves to life on high, cold
elevations, or in dry, hot desert areas, or to migrate via the
land-bridges to more favorable climates. Those unable to do this perished.

"For instance, take the age of reptile dominance, (the Mesozoic Era),
which was in turn divided into four periods, those of dinosaurs, (the
Triassic period, a rock from which I showed you, if you remember), the
Jurassic period, which gave rise to flying reptiles, from which our first
birds were derived; the Comanchean period, which gave rise to flowering
plants and the higher insects, and the Cretaceous period, when our most
primitive mammal forms evolved.

"At first the earth was peopled with dinosaurs and flying dragons, and
the seas by squid-like mollusks. In those days all the earth was level,
swampy, tropic and overgrown with giant tree ferns and a primitive
conifer.

"As the high mountain ranges arose and deserts were made, these forms
gradually gave way to flowers and hardwood forests, peopled with insects
and mammals. Only the most intelligent forms survived, and the struggle
itself developed a higher degree of intelligence."

"What in tarnation were _dinosaurs_?" asked Long Lester.

"Oh, haven't you ever seen pictures of them?" laughed Ace. "Picture a
giant lizard, perhaps 40 feet long----"

"Here, here," protested the old man. "I don't bite."

"It is perfectly true," said Norris soberly.

"Honest Injun!" vowed Ace. "One of these fellows was a sort of cross
between a crocodile and a kangaroo, what with his long hind legs that he
could walk half erect on. There were some as small as eight or ten
inches, too, and some so large that you wouldn't have c