50 in 50

"Correct. Our ecological niche would be gone. The oysters would thrivaand cover the world. If we survive we are all equally fit in the eyes or nature. Now let us see how our animals with skeletons are faring in a new niche. Dry land."

A press, a motionless movement, and they were on a muddy shore by a brackish swamp. Teacher pointed to the trace of a feathery fin cutting through the floating algae.

"The subclass Crossoptergii, which means fringed fins. Sturdy little fish who have managed to survive in this stagnant water by adopting thsir swim bladders to breathe air directly and to get their oxygen in this Jnanner. Many fish have these bladders that enable them to hover at any given depth, but now they have been adapted to a different use. Watch!"

The water became shallower until the fish's back was above the water, then its bulging eyes. Staring about, round and wide, as though terrified by this new environment. The sturdy fins, reinforced by bone, thrashed at the mud, driving it forward, further and further from its home, the sea. Then it was out of the water, struggling across the drying mud. A dragonfly hovered low, landed — and was engulfed by the fish's open mouth.

"The land is being conquered," Teacher said, pointing to the humped back of the fish now vanishing among the reeds. "First by plants, then insects — and now the animals. In a few million years, still over 255 million years before our own time, we have this…"

Through time again, rushing away on the cue word, to another swampy scene, a feathery marsh of ferns as big as trees and a hot sun burning through low-lying clouds.

And life. Roaring, thrashing, eating, killing life. The time researchers must have searched diligently for this place, this instant in the history of the world. No words were needed to describe or explain.

The age of reptiles. Small ones scampered by quickly to avoid the carnage falling on them. Scolosaurus, armored and knobbed like a tiny tank pushed through the reeds, his spiked tail dragging a rut in the mud. Great Brontosaurus stood high against the sky, his tiny, foolish head, with its teacup of brains, waving at the end of his lengthy neck, turned back to see what was bothering him as some message crept through his indifferent nervous system. His back humped-up, a mountain of gristle and bone and flesh and hooked to it was the demon form of Tyrannosaurus. His tiny forepaws scratched feebly against the other's leathery skin while his yards-long razor-toothed jaws tore at the heaving wall of flesh. Brontosaurus, still not sure what was happening, dredged up a quarter ton of mud and water and plants and chewed it, wondering. While high above, heaving and flapping its leathery wings, Pteranodon wheeled by, long jaws agape.

"That one's hurting the other one," Mandi-2 said. "Can't you make them stop?"

"We are only observers, child. What you see happened so very long ago and is unalterable in any way."

"Kill!" Grosbit-9 muttered, his attention captured for the very first time. They all watched, mouths dropping open at the silent fury.

"These are reptiles, the first successful animals to conquer the land. Before them were the amphibia, like our modem frogs, tied un-breakably to the water where their eggs are laid and the young grow up. But the reptiles lay eggs that can hatch on land. The link with the sea has been cut. Land has been conquered at last. They lack but a single characteristic that will permit them to survive in all the parts of the globe. You have all been preparing for this trip. Can anyone tell me what is still missing?"

The answer was only silence. Brontosaurus fell and large pieces of flesh were torn from his body. Pteranodon flapped away. A rain squall blotted out the sun.

"I am talking about temperature. These reptiles get a good deal of their body heat from the sun. They must live in a warm environment because as their surroundings get cooler their bodies get cooler…"

"Warm blooded!" Agon-1 said with shrill excitement.

"Correct! Someone, at least, has been doing the required studying. I see you sticking your tongue out, Ched-3. How would you like it if you couldn't draw it back and it stayed that way? Controlled body temperature, the last major branch on the ever-branching tree. The first class of what might be called centrally heated animals is the mammalia. The mammals. If we all go a little bit deeper into this forest you will see what I mean. Don't straggle, keep up there. In this clearing, everyone. On this side. Watch those shrubs there. Any moment now…"

Expectantly they waited. The leaves stirred and they leaned forward. A piglike snout pushed out, sniffing the air, and two suspicious, slightly crossed eyes looked about the clearing. Satisfied that there was no danger for the moment, the creature came into sight.

"Coot! Is that ever ugly," Phill-4 said.