She had plenty to figure out: her body’s preposterous changes, for one. The fact that all her friends were adults. The mysteries of Greek, which even with the pastor’s help she still found difficult. She had the memory of her father and his work to ponder (his shop, behind the house, remained closed; his tools and machines had mostly been sold but a few remained, which she was teaching herself to use), as well as her own projects and investigations and the need to help her mother and Hester. How could she make some money? The royalties trickling in from a screw caliper micrometer her father had patented, which during the war had suddenly become vital to the manufacture of sheet metal, were all that kept her family afloat, and despite her mother’s economies they were often short. Some girls her age did fancywork but she was clumsy with her needle and instead convinced a few neighbors to let her do boys’ work. She split wood, for which she had a decided knack. Dug manure into garden plots, picked grapes, cleaned chickens. No more babysitting, after Bernard, but she wrote letters for anyone who needed a hand.

School she loved unreasonably, slicing through her studies: botany, geology, natural history in all its forms. As she moved through high school her classmates dropped out one by one, but she graduated with thirteen others and, with a scholarship and her mother’s encouragement, enrolled at Oswego and started training to be a teacher herself. Mr. Williams, who’d helped her even when he couldn’t quite follow her interests, retired just as she finished her training and suggested Henrietta for his job. Reduce every subject to its elements, and present one difficulty at a time, she learned. Proceed step by step.

Over the next five years, after the shock of having much of her schooling upended by her friend Daphne, she wove the ideas of the British naturalist Charles Darwin into her classes, always teaching by way of concrete examples. A dead dog, admittedly overripe, dissected by her students; a comparison of the rumen of a cow with that of a sheep. Aquariums on the benches, terrariums under the windows; seeds sprouting in low metal trays and sometimes a rescued turtle or a fledgling. Each year her students built a pair of simple microscopes and learned to use their forceps and mounted needles, examining the heads growing like flower petals from the hydras they’d slit with sharp blades. They experimented with sticklebacks, raised ants in a thin glass box, and passed around stuffed wrens and fox skeletons and the creatures floating in jars. The year that her sister, Hester, was in tenth grade, they strung together the bones of an owl she’d found, dead but almost entirely intact, in the woods. The year after that, in 1878, her class included the only other person she’d held and fed as an infant.