This is what the Earth must have looked like at the dawn of Early Man. More wilderness than can be charted. More wildlife than can be counted: bears, bald eagles, moose, deer, whales, spawning salmon. Its immensity overwhelms. Glaciers advance and retreat forming ridge after ridge of mountain peaks receding into the distance. In spring, swift streams somersault over 1,000-foot precipices in a headlong rush to the sea. Dense, dark green primordial forests stretch as far as the eye can see. Everything seems larger than life. But it's not just the size, it's how you fit into its gigantic proportions that is the soul- searing beauty of an Alaska cruise vacation. With activities and tours that range from the mild to the wild, and everyone succumbs to the wonder. No wonder they say that once you've been to Alaska, you never really come all the way back.