Well, in fact it is that Darwin's work was the genesis of the modern Theory of Evolution, but Leewenhoeuk, Lamarak and Wallace all had made generally similar theories. Wallace's theory being very similar to Darwin's.
Darwin actually had penned his theory much earlier than Wallace, but he sat on the idea because he feared the reprecussions from a societal/religious perspective.
But when it was clear Wallace was close to discovering and publishing his work, Darwin's associates encouraged him to publish his results.
The "theory" part is trivially simple - which is part of the beauty, has really two parts.
1. That through heredity traits from both parents are combined and passed on in children, this combination causes small changes.
2. For most species, over time, many more organisms are born than can/do survive through life long enough to produce offspring of their own.
That's basically it. The whole theory is basically summed up right there. It might seem obvious, but it really wasn't since the mechanism of change is not specified, and for a long time a divinely inspired answer was considered good enough.
About 3 years after Darwin wrote Origin of Species, Abbott Gregor Mendel living in what would be today Romania but which was then Austria-Hungary, had conducted EXHAUSTIVE studies of heredity and formed the basis of what we consider modern genetics as well as developing the basics for statistics.
This REALLY seriously impressive when you consider that he tracked and analysed the development and collection of over 300,000 pea and other plant specimens.
It took decades, and he did it all without the benefit of computers OR modern statistical methods. What's most amazing was that he was also able to do this through the support of the RC Church both as a junior monk and then as Abbott, of his monestary.
Darwin actually was never able to solve the idea of the mechanism of heredity.
Ironically, Mendel wrote to Darwin, of his research and discovery, sending him his data and published work. Unfortunately , Darwin received the correspondence , but never opened it - because it was among much correspondence from across the globe, and was in German and he was near the end of his life.
In the 1920's with the development of advanced statistical methods, some of the problems which were unable to be solved by Mendel, (due to the complex nature of the math he was attempting), were solved, basically allowing a full mathematical understanding and the underpinning for the process of heredity.
However, the final transformation of the theory into something very similar to a Law of science, was in the investigation into cellular structures in the 1940's post-war period.
Several scientists, Among them Roslyn, Pauli, Crick, White and Watson were all performing competitive, and collaborative X-ray analysis of the molecules in cells, and specifically what would become known as DNA.
Watson and Crick stumbled upon the discovery of the correct interpretation and model of the DNA molecule and with that solved the actual physical mechanism of the evolution.
With advances in genetic science and computing and biological techniques. We today are in the position of being able to use DNA and the theory of evolution confidently to convict criminals of crimes or free innocent men - wrongly accused. Solving crimes, settling paternity, and all manner of other forensic science would be pretty much not possible if the theory was wrong.
The most interesting thing - to me - (it's my actual line of work), is the development of advanced computer models.
The idea behind the theory is wonderfully supported by the facts, the physical evidence and the mathematics available presently, this gives us VERY good evidence to suggest that the theory is correct.
If it is NOT correct, than we know that whatever the proper answer IS, would certainly incorporate - in whole or part, a considerable part of what is known presently.
And the cool part for me is that the last step for Darwin's theory to become Darwin's LAW of evolution, would be to create mathematical models which can PREDICT, the potential outcome of a genetic change.
Practically speaking, we can know with certainty that certain genetic diseases are - and will be caused by certain defects in the genome of an organism - so we are VERY CLOSE to being able to say this scientific principle in in fact a law.
But the real proof will be in a new form of advanced "genetic" mathematics, specifically , artificial life modeling and genetic algorithms. These two fields of research, will be the final mathematical underwriting of evolution as a law of science.
It's also possible that the mathematics for the interactions at such a detailed level are beyond the understanding of humans at a normal level.
It might be possible to construct models for which we cannot properly understand the results, because we don't have a good working knowledge of an underlying meta-mathematics.
Such examples of emergent behavior we do not fully understand exists in many of the experiments that have been conducted since the 1980's - Notably , Avia, Conway's Game of Life and Polyworld.
While we humans may have difficulty understanding these models, it's quite possible that a mathematical / computer model that CAN understand these constructions, could be considered both sentient and smarter than us.
But at the very least, unless a relatively trivially easy underlying formula/law is discovered (like the Theory of Gravity), the understanding of complex computer models designed to perform the possibly trillions of calculations necessary will only be had by humans assisted by computers.
It's important to understand , since many folks do not have a good understanding of the terminology, I'll explain where we are in steps.
1. The theory is proposed in about 1858-59.
2. The mathematics of heredity start to be understood 1840-1920.
3. Watson and Crick discover DNA - 1948-53, theory essentially proved as physically correct and occurring in nature.
4. Advanced genetic techniques allow detailed molecular maps - 1975-2001
5. Hyper-accurate genetic mapping of the genome 2001-present, allowing for specific genome prediction and genetic forecasting of individual organisms.
6. Successful predictive mathematical models , allowing for final mathematical proof of evolution and a general theory which is applicable more or less universally. 1956-present