The distinctive thing in human evolution, on the other hand, is the development of a process which is not fixed but plastic, which adapts itself directly to each particular situation, and is capable of an indefinite number of appropriate and successful modes of action. This happy result involves a change in the hereditary process, as well as the rise of a new process to supplement it. The hereditary tendencies, instead of remaining definite and fixed, have to become vague and plastic in order that they may be moulded into the infinitely various forms of human conduct. The hand-organ has to become a piano, which will yield no tune at all except under the touch of a trained player, but under such a touch is capable of infinite melody. The player, to carry out the analogy, is the human intelligence trained by working with the social environment. This is the agent through which situations are understood and hereditary tendencies organized to meet them. The instinctive life is no longer a mere mechanism as—comparatively at least—it was before, but a plastic thing with a mind to guide it. And this new, distinctively human process implies a complex social life, with a system of communication, tradition, and education; because it is through these that intelligence is enabled to develop and to organize its control. The human process, then, involves a plastic heredity prepared to submit itself to the guidance of environment as interpreted by intelligence. The distinctively human heredity is not an inborn tendency to do definite things, but an inborn aptitude to learn to do whatever things the situation may call for. Just what is it, then, that we owe to heredity? In general it is capacity, or, more exactly, _lines of teachability_. We must depend upon the environment to stimulate and define this capacity, to supply teaching along these lines. When we say that a child is a born musician we mean, not that he can play or compose by nature alone, but that if he has the right kind of teaching he can rapidly develop power in this direction. In this sense, and in no other, a man may be a born lawyer, or teacher, or poet, or, if you please, a born counterfeiter or burglar. Suppose that twin children are born with precisely the same hereditary tendencies, and that one of these is carried off and brought up in a French family, while the other remains with its parents in America: how would they be alike, and how different? Presumably their temperaments, as energetic or sluggish, and their general lines of ability, so far as these found any encouragement, would remain similar. But all definite development would depend upon the environment. The former child would speak French and not English; if he developed ambition the objects of it would be suggested by the life around him, his whole system of ideas would be French, he would enter body and soul into the social process of France. And so it would be if he were taken to Germany, or China.