Big difference.
Maybe in your mind however he constituted theory using the method of science. The method of science is observation of a material fact, evidence and emperical evidence of the fact and a explination of the fact. Theories are composed of facts and laws. It is not the model of evolution.
It's coincidental that his methods resemble the modern approach.
Where he stepped off the planet was with his sexual selection theories ~ although they are still popular, other, more rational bases for the observations keep getting thought up. Remember, for Darwin, long separations from his wife may have given him a lot of ideas. With the animals, it's simply chemistry.