“Certain units of behaviour, such as the crawling and speech of our examples, do not belong to an initial repertoire of operant responses that comes into being with the organism. They do not originally emerge as full-blown behavioural entities in the way that we find unconditioned lever-pressing, for example, occurring at a low but typical rate in Flyer, the chimp. This is why they they appear to be “new”. Their ultimate occurrence depends on the fact that behaviour, in all its features, is variable, and its variations can be reinforced.”

Introduction to the “Essential Works of Pavlov”