"[I]f Amoeba were a large animal, so as to come in the everyday experience of human beings, its behaviour would at once call forth the attribution to it of states of pleasure and pain..."

"... of hunger, desire, and the like, on precisely the same basis as we attribute these things to the dog," wrote Herbert Spencer Jennings in "Behavior of the Lower Organisms," quoted by Oliver Sacks, in an article titled "The Mental Life of Plants and Worms, Among Others," where I learned, that the last book Charles Darwin ever wrote was about worms, "The Formation of Vegetable Mould through the Action of Worms: with Observations on Their Habits."

Sacks says:

Jennings’s vision of a highly sensitive, dog-size Amoeba is almost cartoonishly the opposite of Descartes’s notion of dogs as so devoid of feelings that one could vivisect them without compunction, taking their cries as purely “reflex” reactions of a quasi-mechanical kind.