WHEN a seven-year-old we know heard that Dr. Seuss had passed on, her instant response was, "And to think I last saw him on Mulberry Street." What better insight into the legacy of a man who devoted almost half a century to writing and illustrating books that delighted both children and their parents?The worlds created by Dr. Seuss, a.k.a. Theodor Seuss Geisel, were inhabited by furry, prickly, spindly characters whose passions and concerns only slightly exceeded some of those visible in the "real world." Remember the Zooks and the Yooks, who went to the brink of total war over which side of the bread should be buttered? Between "And to Think That I Saw It on Mulberry Street," published in 1937, and his 1990 book, "Oh the Places You'll Go!," which became a best seller and popular graduation gift, Dr. Seuss gave us Horton the Elephant, Yertle the Tertle, Bartholomew Cubbins, and dozens more. Forty-seven books in all, translated into 20 languages. He also wrote for film, creating the documentary "Hitler Lives" and the cartoon hit, "Gerald McBoing Boing," both Oscar winners. The impact of someone like Dr. Seuss (if there ever could be someone else like him) is immeasurable. How many youngsters, to say nothing of adults recuperating from the day's chores, found themselves refreshed by the antics of Wockets and Sneetches. How many children discovered, through him, that language is infinitely more than a dull classroom exercise? How many kids got an inkling of environmental concern from the somber, mustachioed Lorax, who proclaimed "I speak for the trees!"? Dr. Seuss wrote and drew for children, but his themes were ageless. Everyone was heartened to find that Horton's faithfulness had its reward, or that even a reprobate like the Grinch could change. Will the Seuss tales endure? It was not that long ago, in 1985, that the graduating class at Princeton rose, unprompted, to recite "The Cat in the Hat" when Mr. Geisel was awarded an honorary degree. There's at least one more generation of parents, and probably more, who will make sure their children meet the good doctor.