The regiments of small mammals at the museum of vertebrate zoology roll out smoothly, on long wooden trays, from the steel cabinets of their museum cases. deer mice, jumping mice, trade rats, pocket gophers, moles, voles. they march rank on rank, shoulder to shoulder, all in the same direction, their tails wired stiff and straight behind them. In the library hush and faint echo of the museum, above the range of human hearing, a visitor can almost detect it, some Pied Piper playing his tune. The rodent armies stride in lock-step and brainlessly, the extracted skull of each soldier trailing behind, preserved along with the jawbone in a corked glass vial at the tip of the tail.

In the labyrinth of museum cases, I found the row marked “Soricidae,” the family of the shrews. One cabinet listed Sorex monticolus obscurus, the montane shrew. I rolled out that tray. The shrews are the smallest of mammals, so there was room in this drawer for a lot of them. The tray seemed to swarm. All the hosts of small mammals in the museum’s collection seem hasty, marching along in a kind of arrested quick-time, but these shrews were a half step quicker than the rest. This was a hallucination, of course. The specimens were all perfectly motionless. The hallucination arose, no doubt, from my previous knowledge of the raging metabolism of shrews.

Among the creatures of the Sierra Nevada, a great migration is under way, all in one direction, except for the montane shrew. Sorex monticolus obscurus is no longer confined to the high altitudes where it lived a century ago. The Pied Piper of montane shrews, for motives unknown, is leading them downward, counter to the diaspora of the others. I was curious about this species as an odd little footnote to what is shaping up as the greatest unnatural disaster ever to face us.

In a forward rank on the tray, one montane shrew, a specimen from Kings Canyon, was an albino, and another in the middle of the formation was pale beige. These were the Hawkeye and Trapper John of this battalion—the aberrations—and all the rest were in neat ranks of uniform dark brown. I picked my way down the dark-brown rows, like a general reviewing troops, until I came to MVZ No. 22021. This was the shrew I was after. In black ink on its tag, I recognized the Victorian handwriting of the collector, Joseph Grinnell, the first director of the museum. Grinnell had trapped this shrew, a male, near Mono Meadow, in Yosemite National Park, at an elevation of between 7,300 and 7,400 feet, on June 17, 1915.

In Grinnell’s day, the montane shrew was called the dusky shrew, and its scientific name was Sorex obscurus obscurus. In his field notebook, Grinnell had entered the measurements and other particulars of this specimen: “Sorex obscurus. (Testes large) 112 x 44 x 13.5 x 4. In oat-baited mouse trap under rotten log 5 feet from running water.”

Grinnell was a man preternaturally attentive to the natural world and almost frighteningly disciplined in recording its details. Small field notebooks were standard issue at the museum he founded, and he insisted that all his apostles carry them. Thanks to Grinnell’s field notes, we have not just the context of that rotten streamside log where the shrew met his Maker, but also a clear, concise, almost novelistic account of what the shrew’s last evening was like:

In the night heard a Horned Owl repeatedly—deep-toned, “Who, who-who, who.” At faintest dawn heard the full call of Olive-sided Flycatcher. This was the earliest bird to sing. A little later Wood Pewees began. . . Hylas were very entertaining last night around the little spring near which we made our bed.

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