Thye whole quote is (emphasis mine):

I believe that our Heavenly Father invented man because he was disappointed in the monkey. I believe that whenever a human being, of even the highest intelligence and culture, delivers, an opinion upon a matter apart from his particular and especial line of interest, training and experience, it will always be an opinion so foolish and so valueless a sort that it can be depended upon to suggest to our Heavenly Father that the human being is another disappointment and that he is no considerable improvement upon the monkey.

    - Mark Twain's Autobiography; Mark Twain in Eruption