“I can only compare these great aquatic forests with the terrestrial ones in the inter-tropical regions ,” Charles Darwin once wrote of the giant kelp growths off the Pacific Coast. Rich in delights and trophies for the skin diver as well as potash for industry, the undersea forests are menaced by the eating habits of the spiny purple sea urchin. Mr. Marx, a California free-lance writer, here tells of the mysteries and values of the forests that astonished Darwin.