This is the Law of the Yukon, that only the Strong shall thrive;

That surely the Weak shall perish, and only the Fit survive.

Dissolute, damned, and despairful, crippled and palsied and slain,

This is the Will of the Yukon -- Lo! how she makes it plain!

"Songs of a Sourdough" by Robert William Service.

The first benefits of exercise barely appear before the question intrudes: Why am I putting myself through all this?

If you're a few weeks into your program, any conditioning program, you're out running or enduring some similarly repetitious, boring exercise. Your body has given up active objection and has reluctantly joined the campaign. Perhaps you can actually run the whole first course without collapsing on a bench midway. You can breathe whole lungs full of air.

Even that much has not come easily. It's just that the muscle scream has receded to a grumble; the physical self is quashed by the mind, but now even it threatens to quit unless some very serious meaning can be ascribed to all this effort.

Your head is crammed with odd physiological information -- resting heart-beat rates, lung capacities, oxygen and carbon dioxide transfers, remedies for shin splints, types of bandaging for sprains. You've read books, talked with friends and cynically absorbed the expertise of clerks who peddle expensive sneakers. You know more about physiology and orthopedics than pre-med students did when you were in college.

But what you don't know with any certainty is where fitness starts and stops. When will you be fit? When can you -- gasp! -- stop?

It's good for you, the doctor tells you so. Fit people should live longer, live better. You want this, want to be fit, healthy. You know you want to look better and live longer -- want to live forever actually. You want that better sex life runner-author James Fixx promised. At the moment you would like a little taste of the "runner's high" so talked about, instead of dull aches that don't wash away in the shower.

The questioning doesn't go away. What is fit?

A popular animal ideal of fitness is embodied in "Rocky": all that raw strength and endurance, all that running and one-handed push-ups and lifting weights. Then more running, until the Italian Stallion was inexorable force. Heart and lung, muscle and blood, he had determination that went right through hurt.

Robert Service's less than sublime notion of fitness in the harsh Yukon northwest put more emphasis on the mental and moral aspects of fitness than we do now. Life was considerably more difficult, and no one was exploring in a four-wheel-drive Blazer. Without a certain cardiovascular fitness you couldn't even get there -- much less survive once you did.

Fitness, in the muscular sense, loses meaning looking back across frontiers. In Ancient Greece, in Rome, in the medieval world, fitness was married to war and hunting. Further back, to anthropologist Richard Leaky's earliest men who lived as wild animals on African plains, every moment needed fitness; the very idea of exercise was far in an abstract future.

Recent studies of wolves have shown that they tend to keep the herds of deer they feed upon lean by culling out the weak and crippled. Biologists have witnessed incidents of confrontation between packs of wolves and a healthy, vigorous deer in which the wolves have not attacked and killed when the deed was obviously possible. The same biologists have speculated that the kill might have been too much exertion for the wolves to undertake or, more speculatively, that the wolves simply recognized the deer was not for them -- fit, it deserved to live.

Perhaps our struggle for fitness is a sweaty placebo that allows us to believe we deserve life, living and indulgences. Perhaps fitness is a state of grace -- as well as strength and endurance -- not to be grasped, only pursued.