@Mor - Well, I don't deny it was amazing that Darwin came up with his theory, after doing nothing but observing different adaptions and species distribution on his travels.
But, even if DNA wasn't discovered for a while, genes and their role in changing a species was discovered by Mendel around the same time that Darwin was around. And, if you really think about it, they knew about how genes worked for thousands of years before that.
Any farmer worth the name knew that bloodlines were important and that you could make a better, stronger animal by matching up other strong animals. They had to know how to make different color combinations and so forth. That's how we have so many different breeds within each domestic species now.
I suspect it was never properly studied because scientists considered farmers to be beneath them, which is a shame, really, since genes are the key to a lot of the science that goes on today.