Douglas Adams. The Salmon of Doubt: Hitchhiking the Galaxy One Last Time. New York: Random House, 2002, 135–136.
Also quoted by Richard Dawkins in his Eulogy for Douglas Adams (17 September 2001) http://www.edge.org/documents/adams_index.html
Context: If you try and take a cat apart to see how it works, the first thing you have on your hands is a nonworking cat. Life is a level of complexity that almost lies outside our vision; it is so far beyond anything we have any means of understanding that we just think of it as a different class of object, a different class of matter; 'life', something that had a mysterious essence about it, was God given, and that's the only explanation we had. The bombshell comes in 1859 when Darwin publishes On the Origin of Species. It takes a long time before we really get to grips with this and begin to understand it, because not only does it seem incredible and thoroughly demeaning to us, but it's yet another shock to our system to discover that not only are we not the centre of the Universe and we're not made by anything, but we started out as some kind of slime and got to where we are via being a monkey. It just doesn't read well.