A law means that you start with a null condition, apply certain fixed parameters to that null condition, and as a result, the null condition is modified to a new condition every time the experiment is undertaken. By this definition, evolution will always be a theory since the time span required is enormous, a million years or so.
The odd thing is, history seems to tell us that a very, very large percent of the eminent scientists in the roughly 40-year period from Darwins The Origin of the Species (1859) to the end of the 19th century agreed wholeheartedly with Darwin.
The remarkable thing is, that in the ensuing 110 years (1900 to the present times), scientific knowledge, investigation, available laboratory equipment, etc. have increased by an enormous factor probably 200 to 300 times and all of this increase supports evolution.
Also, we now know, very high frequency electromagnetic radiation (such as X-ray and gamma rays) along with the very small particles of nuclear physics (such as alpha, beta, protons, neutrons and many other newly discovered particles) are constantly bombarding all plant and animal life on earth. This bombardment comes from outer space (mostly our sun) and from the natural radioactive elements in the Earths crust.
All this can cause cancer, as well as DNA damage, which is minor and easily taken care of by white blood cells. But, on rare occasions damage is done, not to the individual, but to his or her reproductive DNA, which ends up modifying the next generation. Most of the modifications are harmful to that new individual, and will disappear through the mating process. But on rare occasions, the modification will be an improvement, and through natural selection, modify the species. Naturally, the mathematical odds of this process are very slim which is exactly why evolution takes millions of years.