We've all been taught to emulate the hare, now along come Drs. Meyer and Rosenman, a couple of cardiologists, to tell us that it's healthier to be like the tortoise. What in Freudian lingo is called compulsive behavior, the authors describe as Type A behavior: a sense of time urgency, free.floating hostility, an aggressive need to be first, etc. Without sounding like a pair of medical quacks, the doctors stress that Type A behavior may be the most important factor in setting loose the biochemical influences that change coronary artery disease into coronary heart disease. They discuss all the other elements which contribute -- hypertension, cholesterol, smoking, physical inactivity, obesity -- yet in a Type B personality (the tortoise) these rarely bring on a myocardial infarction before the age of 70. It's only a theory to be sure, but a sensible seeming one that may at last get close to the heart of the matter.