A little note on Darwin. When we think of the adjective fit we are inclined to think of someone in jogging shorts and other such hideousness. Fit to us means physically capable and the fittest person is the fastest runner, or something of the sort. This is not the sense in which Darwin* used the phrase.
We still use phrases like fit and proper parent or fit for purpose or not fit to hold office, and that was the original meaning of the word fit. It meant appropriate. The fittest word, is the word most appropriate to the situation.
As Stephen Jay Gould once remarked, it doesn't matter how wonderfully developed a fish is, if all the oceans dry up it will die. Similarly, Darwin couldn't have cared less whether a Galapagos turtle could run fast, only that it was well suited to its environment.
If the cap fits, it doesn't mean that it's been to gym.
How did the word get from one to the other? On horseback. Or more precisely, via horse racing. If a horse was in fit condition for a race then it was fit in the modern sense.
And fiddles? Nobody is quite sure why fiddles are fit, although it is certain that the older sense was being used. There also used to be a phrase as fine as a farthing fiddle, which makes more sense and is more fun to pronounce.
The Inky Fool contemplating the gym
*The phrase pops up in the fifth edition of On The Origin Of Species, although it was in fact invented by a chap called Herbert Spencer, who was trying to describe the ideas set out in the first edition of On The Origin Of Species. So it's Darwin adopting a paraphrase about Darwin, if you see what I mean.