Why St. Denis Should Be the Patron Saint of Evolutionary Theory

Pain is a great teacher, I tell audiences. Take it from me. If PZ Myers hadn't designated April 7 "Paul Nelson Day" a few years ago, to lampoon me for my failure to explain the concept of "ontogenetic depth," I would never have learned just how intractable the problem of animal macroevolution would turn out to be.

"Oh, come on, Paul -- you're exaggerating," says the skeptical reader.

Not really. Before PZ's critique, ontogenetic depth (OD) seemed pretty obvious to me. The metric could be cal...