All right...
That Hundredth Monkey thing again...
As mentioned in Ken Keyes' book above, the Hundredth Monkey Phenomenon is derived from scientific observations conducted in Japan in the 1950s and their subsequent interpretation by Lyall Watson in 1979.
For those too lazy (or too busy) to read the "official" account at the beginning of his book (see other wu above), it goes something like this:
one day, a few monkeys stuck on some Japanese island somehow discover that washing their sweet potatoes before eating them considerably improve the culinary experience
little by little, more and more monkeys learn this new trick from each other, albeit rather slowly
and then, all of a sudden, a certain threshold seems to be reached (an "ideological breakthrough") and ALL the monkey start washing their potatoes overnight
more than that: monkeys on other islands and on the continent with no means of physical contact also suddenly get in this habit
Obvious conclusion: once a certain acquired knowledge has spread past a rather precise threshold (let's say, after "the Hundredth Monkey" learns it), it suddenly becomes part of some collective unconscious, transmitted to the whole species without any need for communication (an extreme case of "pre-cultural" transmission of behaviour).
Additionally, if you are a New Age spiritualist in need of scientific justification for your dopey theories: the notion of "raising consciousness through reaching critical mass" definitely sounds cool especially if monkeys prove it's true (note: I have really nothing against New Age spiritualist and their hokey pseudo scientific theories: in fact I totally love crystals myself, they make all kind of cool reflection patterns really fun to look at, especially when you're on drugs).
Surely, you have heard this story once in your local circle of friends and either dismissed it as yet another stupid hoax or just left the conversation with renewed admiration for the many wonders of Mother Nature.
That's how it came to my ear... And I did piss off the friend telling me the story, not only by dismissing it as pure urban legend but also by openly laughing at his additional bits of theories involving some kind of "read-only, write-protected knowledge DB" accessed by the monkeys through their own wireless "private network" of sort (try to sustain, without laughing, the disturbing vision of hordes of monkeys shoving an RJ-45 up their arse in order to connect to the mothership)...
We were quite drunk, as is customary for such kind of discussion, but he was so vehement in his argumentation, that I did some research on the topic the day after. Quite to my surprise it turned out to be a well documented, detailed, real-world theory... But much less surprisingly, it apparently did not get any kind of scientific attention.
Now I would recommend you stop reading this right now if you don't like spoilers or if you enjoy nice fairy tales with true happy endings... Because as it turns out, this "theory" is a complete hoax:
Although there really were some studies made in the Koshima Islands involving monkeys and strange learning patterns, the whole bit about pre-cultural spreading of knowledge and transmission to other islands was entirely made-up.
According to Watson, his sources were gathered from "personal anecdotes and bits of folklore amongst primate researchers" as well as "off-the-record conversations"...
The problem is that none of the Japanese scientists involved in the original experience (or any other scientist for that matter) backs-up this claim. The whole thing has been officially debunked in the Summer 1985 issue of the Skeptical Inquirer (damn rationalists: won't they let us have our real-world sci-fi fantasies sometimes).
So much for this seducing theory and its heavy use in the eponymous book by New Age writer Ken Keyes (see WU above). Penny Gillespie, who was married to Ken Keyes and had collaborated to his writing, retreated safely by saying this was after all to be read as a "legend, or phenomenon", merely "illustrating" proven concepts such as morphogenetic fields and critical mass... But basically, it is neither a legend nor a valid theory: it is a full-fledged hoax, or, more exactly: a perfectly dishonest and incorrect scientific study. And even though Keynes was probably in good faith, he definitely did a major disservice to his cause by relying on unchecked, plain inaccurate, scientific materials.
It is funny to see how, despite easy availability of debunking material on the net, even the most scientifically minded do not seem to have a problem taking this "theory" at face value, blatantly disregarding basic rules of Physics in favor of some rather shaky (euphemism) pseudo-metaphysical explanation.
Please note the existence of similar theories successfully developed in other disciplines. One simply has to read about Plato's theory of ideas or Jung's concept of collective unconscious to find interesting intellectual stimulation without the flawed pseudoscientific approach used here.