The way Darwin came to his final theory of evolution is a long story. As you know, he published his book "Origin of species" only in 1859, when he was already about 50. However, the first ideas on this issue began to appear in his mind much earlier, and it took more than 20 years of thinking and reflecting on this idea. His views had been forming even in the days of sailing around the world aboard the "Beagle".
Darwin realised that there had to be a natural mechanism of the evolutionary development of organisms that did not require the involvement of additional substances that just had to be due to the fact that there were heredity and variability. It is worth noting that in those days no one knew of the principles and nature of heredity, and scientists and researchers had to guess and poke a finger into the sky.
If there are heredity and more or less random variability in the characteristics of the organism, these hereditary traits affect survival and efficiency of reproduction; there should be a spontaneous process that Charles called natural selection. In other words, organisms with certain genetic variations will leave more offspring, and the other -- correspondingly less. Thus, those characteristics will multiply that contributes to survival and reproduction.
In the opinion of Darwin, in nature, there must be a process similar to the situation in the pigeon loft, where new breeds of pigeons are bred. But in this case, the selection is driven by human desire, i.e. the breeder, while nature also directs some individuals to leave more offspring, and others – produce less because of their genetic differences. It was easy and clear to any Englishman of that time, as the gentlemen were utterly fond of breeding out new breeds of pets: dogs, birds, horses, etc. and everyone knew that by breeding you can easily change the characteristics of organisms that finally will breed new breeds and types. So Darwin called this phenomenon natural selection. Thanks to breeding, scientific evidence of organic evolution are no longer required.
No doubt that over the last 150 years the science has come a long way. On the one hand, humanity turned out to understand the evolutionary process more accurately. On the other hand, modern biologists are still getting amazed at the fact how much Charles had discovered having no idea about genetics and the physical nature of heredity.
The way Darwin came to his final theory of evolution is a long story. As you know, he published his book "Origin of species" only in 1859, when he was already about 50. However, the first ideas on this issue began to appear in his mind much earlier, and it took more than 20 years of thinking and reflecting on this idea.