Darwin articulated the theory of evolution by natural selection.
It goes like this:
Individuals in a community vary in their physical properties. Some may be larger or smaller, faster, stronger, or smarter.
This variability can affect the chances for those individuals possessing more favorable characteristics to survive and reproduce better than the other variants.
If those physical properties are inherited, then the ones who reproduce more leads to having the next generation of their population having more individuals with that property.
What it boils down to is that evolution is the change in the gene pool of a population over time. Natural selection was the term coined to mean that nature breeds animals and plants just like farmers do - it chooses based on the characteristics it “wants” - by which I mean the ones who survive and reproduce more successfully.
So far, so good.
The problem is that while Darwin was working that out, biology still held to a model that we call “blending inheritance.” They knew that offspring could inherit traits from both parents, and they thought the result was a blend of the two - a short person having a kid with a tall person would produce a medium sized kid, and so on.
Blending is completely incompatible with Darwin’s theory, and he worked really hard to fit that square peg into the round hole. He never did.
The irony is that during this same period Mendel was doing breeding experiments on pea plants, crossing tall with short, or different colors of flowers, and so on. He demonstrated that inheritance is particulate. Rather than a blend, it was a random mixture of alternate individual genes that caused the variety in the offspring. Basically, if you cross a plant with wrinkled peas with one with smooth peas, half the offspring will be wrinkled and half smooth. You don’t wind up with plants that had semi-wrinkled peas or which have a mixture of both. Unfortunately for Darwin, Mendel’s work remained undiscovered for decades. (I’m not sure whether wrinkled is actually a 50/50 or not, I’m just using that as an illustration).
It wasn’t until the 1940s that the two biggest ideas in biology were brought together in what’s called the modern synthesis. That’s where we began what could be considered modern evolutionary biology.
Mendel was extremely fortunate to have been working with plants that showed dominant and regressive characteristics so clearly.