A noteworthy feature is the diversity of animal species considered. In an era when animal research was dominated by rats, Washburn references, "not fewer than 100 species, including ants, bees, caterpillars, cats, chickens, chubs, clams, cockroaches, cows, crabs, crayfish, dogs, dragonflies, earthworms, elephants, flies, frogs, goldfish, grasshoppers, guinea pigs, horseshoe crabs, jellyfish, lancelets, leeches, mice, minnows, monkeys, pigeons, pike, planarians, potato beetles, raccoons, salamanders, sea anemones, sea-urchins, shrimps, silkworms, snails, spiders, tortoises, wasps, water beetles, and (yes) rats." Indeed, she devotes an entire chapter to the mind of the simplest animal, the amoeba.