Not initially, and indeed that was one of the major early criticisms of his theory. But he learned of Mendel's work after publishing his own - The Gene has a nice history of this.
Edit: I think you're focusing on the gene part too much. What's important is that inheritance is discrete, regardless of what actually carries it, rather than continuous. Mendel proved that part.
Edit edit: scrap that part about Darwin I misremembered
At the gene level it is discrete yes. It isn't at specimen scale, and appears more continuous.
It's very possible ancient Greeks didn't understand the discrete aspect, it doesn't mean they didn't know in some ways more than we know of evolution. I would assume they did given the numerous incredible polyglot thinkers we find traces of.
Traces, it would seem the biggest part of ancient times writings are gone. And why just thinking of the Greeks, so many civilisations have vanished, many which the left over of their produces confuse us. More civilization disappeared than the count of those we know had existed.
The food stocks, cattles and other pets that accompany us today were the result of non natural selections spanning many human generations. To think civilisations that were able to accomplish that didn't have, in a way, a more comprehensive understanding of natural selection is pretty condescending, or naive of our own understandings.
Sure they probably didn't come up with Crisp, they may not have been able to observed the structures of DNA, they may not have even known how bacteria looked like. Given the challenge I'm in admiration for their findings given how blind we assume those people were.
No, Darwin never learned about Mendel's work, even though Mendel sent him a copy (which was well after Darwin had published the Origin). The copy in Darwin's library has its pages uncut -- meaning that Darwin never got around to even opening it. Darwin instead believed in blending inheritance rather than discrete units.
As for Mukherjee, while his Emperor of All Maladies about cancer was brilliant, The Gene (and Mukherjee's New Yorker article that he expanded to make his book) has a lot of issues as many geneticists and molecular biologists such as Walter Gilbert and Tom Maniatis pointed out at the time.
Thanks for the correction, I must have remembered that wrong. In that case Darwin didn't have the full theory yet either I guess. But my point about the ancients stands.
I agree, we should celebrate the ancients for how much they discovered despite the obvious difficulties.
On your point about things being more continuous on a species scale - two things can be true. Natural selection acts on allele ls of each gene, which are discrete things. But it's also true that large numbers of genes acting in cooperation can produce a continuous spectrum of phenotypes.
Edit: I think you're focusing on the gene part too much. What's important is that inheritance is discrete, regardless of what actually carries it, rather than continuous. Mendel proved that part.
Edit edit: scrap that part about Darwin I misremembered