What the term Natural Selection Really Means
For many years before Darwin, Evolution had become an accepted possibility as an explanation for how various animals developed.   There was clear evidence that, for example, an Arabian Horse so similar enough to an African Zebra to assume they there was a common link.   But having the assumption required an explanation and people were puzzled by what caused the link and by what created the differences.   So there WAS a theory that a horse evolved from a Zebra (or vice versa) but no one could explain HOW.  

Many scientists had the same idea as to how to explain it.   It was easy to imagine that if an animal had an offspring that, simply by chance had some trait that made it superior to the others, it would have a better chance to survive and might, perhaps, pass that advanced trait to it's own children.   At the time, the genetics of inherited traits were not understood as they are today, but they did know that blond parents had more blond children than dark haired parents, things like that, so the germ was there.  

Charles Darwin voyaged on the HMS Beagle to the remote Galapagos Islands of what is now Equador where he saw familiar species, but yet were drastically different and he looked at this and came to the conclusion that IF these animals had once had identical ancestors, then perhaps it was their surroundings that encouraged them to somehow develop differently.   Later his theories could be equally applied to any other area of the world, it was only that the Galapagos Islands represented such drastic contrasts and examples.   So Darwin reasoned that there was an unintentional 'partnership' between animals and nature and that nature forced animals to adapt to their surroundings ...   or perish in the face of the ones that CAN.  

Darwin's original book, titled On the Origin of Species and the Favoring of Races coalesced his thinking and that of others with the stark examples noted from the Galapagos and other places around the world.   Needless to say, it was met with mixed reviews.   Some scientists loved it because it explained what was seen, other scientists, including a few that had corresponded with Darwin on this very subject but who were not credited were a tad miffed, and the Theologians, Flat Earthers and such were outraged.   Generally, the populace of the day didn't give a damn one way or another.  

The political climate of the day DID force subsequent editions of the book to be changed, and toned down in some areas of his argument, so if you read, try to find a used copy of "On The Origin of Species, a Facsimile of the First Edition"