Kropotkin’s actual argument is far more interesting. Much of it, for instance, is concerned with how animal cooperation often has nothing to do with survival or reproduction, but is a form of pleasure in itself. “To take flight in flocks merely for pleasure is quite common among all sorts of birds,” he writes. Kropotkin multiplies examples of social play: pairs of vultures wheeling about for their own entertainment, hares so keen to box with other species that they occasionally (and unwisely) approach foxes, flocks of birds performing military-style maneuvers, bands of squirrels coming together for wrestling and similar games

Spencer, in turn, was struck by how much the forces driving natural selection in On the Origin of Species jibed with his own laissez-faire economic theories. Competition over resources, rational calculation of advantage, and the gradual extinction of the weak were taken to be the prime directives of the universe.

the phrase “survival of the fittest” — usually considered central to the theory and often attributed to Darwin — instead came from British philosopher Herbert Spencer, and didn't appear until the fifth edition of the text.