DICK VAN DYKE... arrived on Broadway in THE BOYS AGAINST THE GIRLS in '58.  His unique physicality, all jutting elbows and knobby knees, caught the attention of Gower Champion who offered him the lead in BYE BYE BIRDIE. Gower saw through his vigorous denials of any real dance acumen with a simply and confidently stated, 'you can dance'. Dick danced his way to his first Tony Award and directly to the first and glorious "The Dick Van Dyke Show".  Five years and five Emmys later... jump ahead thirty years and find the show still on the air, a national treasure.      "Dick's body takes to dance very naturally", says DeeDee Wood, who with Marc Breaux choreographed MARY POPPINS and CHITTY CHITTY BANG BANG and many television specials for Dick over the years. Seeking deeper answers we discover his deep appreciation of Stan Laurel and other clowns.  The suggestions of eccentric legomania and the perfected pratfalls are all facets that reveal, click.  Add mime to this mix and the six years he spent touring the nightclub circuit with his boyhood pal, Phil Erickson, as 'The Merry Mutes' and the picture grows clearer. Dick is a DANCER and a GYPSY.      Dick has appeared in many, many films from SOME KIND OF NUT to THE RUNNER STUMBLES and television movies like the self-revealing THE MORNING AFTER and THE COUNTRY GIRL.  Not in any particular order I must mention three season of 'The New Dick Van Dyke show' and thirteen 'Van Dyke and Co.' variety specials which won him his fourth Emmy.  In '96 he was elected to television's Hall of Fame.      Currently in its seventh season, DIAGNOSIS MURDER stars Dick as Dr. Mark Sloan who is not merely a brilliant M.D. but a very clever crime-solving sleuth who, finding an empty hall in his hospital, practices his tap-dancing.  I don't think there is anyone in the world who could make this more believable than Dick Van Dyke, and the pleasure he seems to find in the assorted adventures makes for most pleasurable viewing.