> What about, "Why is our root metaphor for memes viruses rather
> than genes?"
I'll take a shot at a quick answer. Because memes are (often) propagated horizontally rather than vertically. It is the difference between infection (horizontal) and inheritance (vertical). Of course we can also "inherit" memes from our parents, but then again we can also inherit viruses. Moreover, the closer you examine viruses and genes, the fuzzier the difference becomes. Viruses are little more than a package of genes. This answer is turning out to be not so quick after all ... :-)