In Dawkins's example, "fitness" is defined as "being closer to the original text". "Fitness" in nature is a bit more slippery - essentially, we can define traits in terms of their adaptiveness to their environment. If a trait is adaptive, it is more likely to be passed on than a trait that is dysfunctional, for the simple reason that creatures who possess adaptive traits are more likely to survive than creatures burdened with dysfunctional traits.
And of course, dysfunctional traits can be a relative matter. Consider a pack of predators, some of which are slightly nearsighted, and some of which are very nearsighted. Now, in such an environment, being slightly nearsighted is probably going to be advantageous, compared to being very nearsighted, even though neither animal has what we might call "good" vision. And over time, if better eyesight leads to more successful predation, and it's not hard to imagine that it might, then the animals with "better" eyesight will be better fed, and tend to live longer and produce more offspring than their weaker-eyed counterparts, in spite of the fact that even the animals with "better" vision aren't exactly possessed of incredibly good eyesight. It's sort of like the old safari joke - if you and I are walking through the African veldt, and a lion comes upon us, I don't really have to be faster than the lion in order to get away. I just have to be faster than you ;)
So it's all relative. But what happens if we toss two completely different traits into the mix? Imagine that we have an environment where there are two large predators competing for the same food - the tigers, which have excellent eyesight, and the bears, which have an excellent sense of smell. Assuming their abilities are approximately equivalent in other ways (for the sake of argument), which trait will be "better"?
Of course, there's just no way to know that in advance. Both traits will give the respective animals the ability to detect prey over distances, but which one will be "better" is something that you'd pretty much have to wait and see. You could make an educated guess, but that's all it would be. Basically, in the natural world, "better" and "more fit" have a way of defining themselves, since they're dependent on the environment in which an organism lives. For example, if the area is particularly foggy, good eyesight won't be as helpful as a good sense of smell. But in some other environment, good eyesight might be more adaptive than a good sense of smell. And so forth...