From Darwin -
"Instances could be given of similar varieties being produced from the same species under external conditions of life as different as can well be conceived; and, on the other hand, of dissimilar varieties being produced under apparently the same external conditions. Again, innumerable instances are known to every naturalist, of species keeping true, or not varying at all, although living under the most opposite climates. Such considerations as these incline me to lay less weight on the direct action of the surrounding conditions, than on a tendency to vary, due to causes of which we are quite ignorant."
Even he admits there is a lot he doesn't understand and of course weakens his theory.
150 years ago...
no DNA structure, no Mendelian genetics
everything learned since then has strenthened, not weakened, the theory.
It proves how smart Darwin was. The unknown he was referring to was genes and DNA. He predicted their future discovery. One of the great missed opportunities of science was that Darwin didn't know about Mendel's work, even though it existed at the time.
The ability to predict future discoveries is the hallmark of a solid theory.