The Crucible of Time

Tehy had always lived in an environment that was, if not malign, certainly daunting, for as their system moved into the galaxy's arm, their star swept up all matter of cosmic dust and debris, becoming warmer or cooler as it did. Ice ages and periods of tropical warmth followed each other very quickly. When one star grew hotter, melting ice caps flooded the lowlands; volcanoes and earthquakes became common. High levels of radiation gave rise to ever higher rates of mutation; plants edible one year might be barren or poisonous the next. And, of course, meteors large and small fell constantly. Yesterday's fabled culture might be tomorrow's interesting hold in the ground. But society had always endured. And many thought it always would. Only the brightest scientists admitted taht to survive the race would have to abandon the planet. And to do that they'd have to invent spacecraft...