I thought the listed passage from The Sea-Wolf made my point clear. Here is the key point again;
But,—and there it is,—we want to live and move, though we have no reason to, because it happens that it is the nature of life to live and move, to want to live and move. If it were not for this, life would be dead. It is because of this life that is in you that you dream of your immortality. The life that is in you is alive and wants to go on being alive for ever.
"Natural Law" gives me the right for Survival. The Theory of Evolution/Survival of the Fittest lays out the environment/circumstances which control/modulate this but the instinct to Survive is inborn in all living organisms.
Another passage from The Sea-Wolf which drives home the point;
[Hump] “God made you well,” I said.
[Wolf Larsen] “Did he?” he answered. “I have often thought so myself, and wondered why.”
“Purpose—” I began.
“Utility,” he interrupted. “This body was made for use. These muscles were made to grip, and tear, and destroy living things that get between me and life. But have you thought of the other living things? They, too, have muscles, of one kind and another, made to grip, and tear, and destroy; and when they come between me and life, I out-grip them, out-tear them, out-destroy them. Purpose does not explain that. Utility does.”
“It is not beautiful,” I protested.
“Life isn’t, you mean,” he smiled. “Yet you say I was made well.
...
“Stability, equilibrium,” he said, relaxing on the instant and sinking his body back into repose. “Feet with which to clutch the ground, legs to stand on and to help withstand, while with arms and hands, teeth and nails, I struggle to kill and to be not killed. Purpose? Utility is the better word.”