If there’s no need for movement, then no need for a brain, I’ve learned, a fact demonstrated by the sea squirt, a small creature that swims freely in its youth until it settles on a rock. Then it devours its own brain. And spinal cord. It simply doesn’t require them any longer. (God, don’t let me settle.) Need for movement leads to need for muscle. The brain evolves in order to plan and execute reaching, grasping, turning, according to the expert on Charlie Rose, which I watch on my iPad while walking on the treadmill to rebuild my strength. Plenty of species thrive without brains, he says. It could be different on another planet, I suppose, but here evolution of the brain is about muscle. Just ask Arnold Schwarzenegger or an evolutionary biologist. Yet the brainless sea squirt still gets upset, still squirts. Maybe it’s innate, like a horse’s hide shuddering to dislodge a fly. Maybe that’s why I started moving and arranging boulders last fall. I thought I was making a terrace. But afterward it looked more like a grave.