With them, being tree-dwellers, the liability of falling was an ever-present menace.

Many lost their lives that way; all of them experienced terrible falls, saving themselves by clutching branches as they fell toward the ground.
Now a terrible fall, averted in such fashion, was productive of shock.
Such shock was productive of molecular changes in the cerebral cells.
These molecular changes were transmitted to the cerebral cells of progeny, became, in short, racial memories.

Thus, when you and I, asleep or dozing off to sleep, fall through space and awake to sickening consciousness just before we strike, we are merely remembering what happened to our arboreal ancestors, and which has been stamped by cerebral changes into the heredity of the race.
There is nothing strange in this, any more than there is anything strange in an instinct.

An instinct is merely a habit that is stamped into the stuff of our heredity, that is all.

It will be noted, in passing, that in this falling dream which is so familiar to you and me and all of us, we never strike bottom.