I think that Ernst Mayr's treatment of the question is so logical, so concise, and so absolutely correct, that I could not imagine a better graphic than one that uses his words. I did not add the slight clarification of the last point, that even if such life exists, it is probably so far away that we could never have contact, but I hope the layout conveys that notion.

The facts are very simple, the fantasies are not convincing, although they are sometimes, as in Kubrick's 2001, stimulating. The only other meditation on the likelihood of extraterrestrials that I have found at all compelling is Fontenelle's Conversations on the Plurality of Worlds, but he wrote that in 1686, without the benefit of modern biology or Darwin's accomplishments.

Gary