At dinner last night a friend in his mid 50s asked, quite genuinely: so what exactly is a meme? Our teenage son laughed about this apparent ignorance but the confusion is well founded given that many people today use the word to mean a short lived fad on the internet, as compared to the original usage as a unit of meaning that is culturally transmitted and self-replicating (hence the coinage which is based on “gene”). The two usages are obviously related and yet also different. In particular,...