We have been considering which species was on the whole most finely equipped to be rulers, and thereafter achieve a high civilization; but that wasn’t the problem. The real problem was which would do it: a different matter.

To do it there was need of a species that had at least these two qualities: some quenchless desire, to urge them on and on; and also adaptability of a thousand kinds to their environment.

The rhinoceros cares little for adaptability. He slogs through the world. But we! we are experts. Adaptability is what we depend on. We talk of our mastery of nature, which sounds very grand; but the fact is we respectfully adapt ourselves first, to her ways. “We attain no power over nature till we learn natural laws, and our lordship depends on the adroitness with which we learn and conform.”

Adroitness however is merely an ability to win; back of it there must be some spur to make us use our adroitness. Why don’t we all die or give up when we’re sick of the world? Because the love of life is reenforced, in most energized beings, by some longing that pushes them forward, in defeat and in darkness. All creatures wish to live, and to perpetuate their species, of course; but those two wishes alone evidently do not carry any race far. In addition to these, a race, to be great, needs some hunger, some itch, to spur it up the hard path we lately have learned to call evolution. The love of toil in the ants, and of craft in cats, are examples (imaginary or not). What other such lust could exert great driving force?

With us is it curiosity? endless interest in one’s environment?

Many animals have some curiosity, but “some” is not enough; and in but few is it one of the master passions. By a master passion, I mean a passion that is really your master: some appetite which habitually, day in, day out, makes its subjects forget fatigue or danger, and sacrifice their ease to its gratification. That is the kind of hold that curiosity has on the monkeys.