of those periods now. The organic world is being
stimulated to develop even better bodies, endowed with even more alert
brains.

"Life is easiest of all for the inhabitants of the ocean. That is why
they have developed so little intelligence."

"Is that why it's such an insult to call any one a poor fish?" grinned
Ted.

"An ichthyosaurus?" supplemented Ace.

"As has been said before," Norris took up the thread of his talk, "with a
drier climate and soil, comes the need of developing a faster mode of
locomotion, for food no longer lies or swims everywhere about, as it did
in the ocean, and in the swamps, and tropic humidity. Food and water are
scarce, and it is the speediest animal that fulfills his needs. This
speediness on his part means that he uses up more energy, and hence needs
more food, and he needs to assimilate it faster. In other words, it means
increased metabolism. This in turn means that he keeps his body at a
higher temperature. He needs it too, now, with the increased cold. This
results in the development of warm blood, by which the animal can
maintain his body warmth regardless of winter cold. If it had not been
for conditions that forced certain reptiles to develop warm-bloodedness,
we would have no birds or mammals to-day, for as you doubtless know,
birds and mammals both were evolved from reptiles."

"I swan!" was all the old prospector could say.

"Yes, the first mammals developed from a reptile known as the cynodont.
Many of these reptiles had long legs and could travel with the body well
off the ground. Birds originated from the same reptilian stock as did the
dinosaurs. First their hind-legs grew long so that they could run on
them,--and you will notice at the Museum how the legs of a dinosaur are
joined to the body exactly like a bird's,--then their scales gradually
evolved into feathers.

"There is a lot more to it than I can tell you now, but after various ups
and downs, dinosaurs became extinct and Nature tried out several kinds of
warm-blooded, furry mammals, some of them herbivorous and built for speed
to run away from their enemies, some of them swamp-dwelling monsters with
heavy legs and small brains, who, slow of movement, relied on horns and
other armor and sharp teeth for their defense.

"But there is no end to this subject. I only mean to make the point that
it was geological changes that drove the fish to land, and the land
animal to higher forms, till finally other geologic