From http://en.wikipedia.org/wiki/Experiments_on_Plant_Hybridization

"In 1936, the statistician R.A. Fisher used a chi-square test to analyze Mendel's data and concluded that Mendel's results with the predicted ratios were far too perfect, indicating that adjustments (intentional or unconscious) had been made to the data to make the observations fit the hypothesis"

Of course this does not diminish the importance of his work. Here's his paper translated to English:
http://www.mendelweb.org/Mendel.html
(By chance this is on my reading list to finish before the end of the year.)

It's worth noting that Darwin's Origin of Species was published in 1859, prior to Mendel's work. From http://www.gutenberg.org/cache/epub/1228/pg1228.txt

"The laws governing inheritance are quite unknown; no one can say why the same peculiarity in different individuals of the same species, and in individuals of different species, is sometimes inherited and sometimes not so; why the child often reverts in certain characters to its grandfather or grandmother or other much more remote ancestor; why a peculiarity is often transmitted from one sex to both sexes or to one sex alone, more commonly but not exclusively to the like sex."