Darwin was not a physicist and had he been one he would have never said this. A slow gradual change implies one genetic change at a time. A quantum leap or abrupt change would require 2 or many more simutaneous and unlikely genetic changes or a million to one chance against. But when popultions number in the millions, the quantum leap or abrupt sudden and dramatic change can occur.
The improbable event is why we can not make hydrogen burn like the sun. You need to get 4 hydrogen atoms to collide simutaneously to get a helium out which is really hard to do which is why we do not have controlled fusion power. The sun does it by have trillions and trillions of hydrogen atoms colliding and a few produce the 4 body collision.
The quantum leaps in evolution would require many genes simultaneously changing. It is unlikely for one individual but one out of millions could make it happen.
That is exactly my point. One out of a million...it would take time for one mutation to be propogated into an entire species, so it would seem that in one layer of rock you would see only a few examples of a fossil with the new mutation and gradually, the numbers would increase as the mutation was procreated. That doesn't seem to be the case. instead, fossil records show up suddenly and in large numbers. The other thing that is weird is that when they look at the earliest fossil records (almost 4 billion years old) the life on this planet was not that diverse, yet only a short time later, there are thousands of different species on record each in significant numbers with no clear examples of in between species. The differences between even the most simple life forms are still so significant that it is hard to imagine that you could go from one to the other without a step in between. So the question remains: where are all the missing links?