Imagine you were a livestock man in medieval England a thousand years ago. It’s early spring. Snow on the ground, mud in the cow lot. You walk the small pasture where the heavy heifers are kept. It’s hard to see much with just the moonlight. But you spot one that’s down in a swale. She’s on her side in the process of calving. One foot is showing.

You check the rest of the cows the best...