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conditions become more hospitable, the species proliferates. 22
    I mention niche opportunities because an analogous phenomenon may be recognized in individual experience. If you once learned to ride a bicycle, you will always be able to do so, provided you suffer no disabling physical change. Being away from a bike for years doesn’t prevent you from knowing what to do once you recycle. The knowledge you have for bike riding is there all along and can be reawakened. Likewise for other skills. 23
    The ability to re-engage skills that have lain dormant for years seems at first to be inconsistent with a prediction that might be made by applying the theory of natural selection to individual brains. That prediction is captured by the familiar phrase “use it or lose it.” If mental representations are untapped for long periods, the Darwinian account would seem to suggest that they should die. The phenomenon of niche opportunities shows, however, that, in the wild, species may “bide their time” if conditions for their survival are not too unfavorable. So too may neural ensembles that support long unpracticed skills, like long-ago biking or erstwhile skiing. As long as the neural ensembles supporting such skills are not crowded out by other competing elements, they can remain viable.
No Stone Goes Unturned
    A last point about evolutionary theory that’s central for what’s to come can be called the “no-stone-goes-unturned” principle. This is not a phrase used by evolutionary biologists to capture a core principle in their field, as far as I know, but it’s one that, for me, captures the essential idea that nature is virtually perfectly efficient and that, by implication given the analogy I’m pursuing here, what’s good for the global goose (species) is good for the individual gander (the individual mind).
    What I mean is that no potentially habitable niche goes unoccupied. Wherever living things can possibly live, they do so—under stones, under eaves, atop mountains, in the hottest deserts, in the coldest coves. Even in places where you’d least expect life to flourish, life can be found. The most salient example I know of are hydrothermal vents unfathomably deep in the sea, where there is scarcely a photon from the sun. Hydrothermal vent wormslive there, comprising a life form no one knew about until its recent discovery by a deep-diving sub. So hardy are the life forms on Earth that pains are taken by NASA to ensure that no bacterium goes along for the ride to other worlds, lest extraterrestrial neighborhoods get infected by terrestrial bugs.
    There’s a tie-in between the no-stone-goes-unturned principle and brain use. A claim in the popular press is that we use only 1/8 of our brains, or some such fraction. Admittedly, some people act like they use less brain power than they should. But that’s different from saying parts of their brains lie idle. Wherever neuroscientists have looked, they’ve found brain regions that are busy. As long as healthy neurons are present in an area of the brain, neurons studied there have been shown to be active. Finding a healthy region of the brain that’s completely dormant is as likely as looking under a stone and not finding a weevil, worm, or wily bacterium. The trillions of living things on Earth find places to live in every nook and cranny. If a “Vacancy” sign appears anywhere in the outer jungle, it doesn’t stay up for long. Vacancies are filled in the blink of an eye. The brain, too, provides a welcome environment for opportunistic bands of neural gnomes to flourish, as long as the living conditions aren’t too difficult. Given the hospitable environment of the brain, a wondrous diversity of neural, and then mental, life can spring up.

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Fighting Neurons, Friendly Neurons
    If it’s a jungle in there and you’re curious how the jungle works, you’d better screw up your courage, put on our hiking boots, and grab your walking stick. Brace yourself for the