James Mark Baldwin, answered from the texts and cited to the page.
How does the child come to do what it could not do before — to speak, to draw, to handle the world as its elders do? Not by invention out of nothing, and not by mere instinct, but by imitation, which I hold to be no trivial aping but the very engine of mental growth. The infant begins with the circular reaction: a movement that happens to reproduce its own stimulus — the hand that strikes and is seen and struck again — and so is repeated, fixed, and refined, the organism holding on to the interesting result.1
Upon this the child builds by imitating first its own acts and then the acts of others, taking the model of another's performance and reproducing it, and in the effort producing something new in itself.2 Imitation is thus the bridge by which the social world enters the child and by which the child's own powers are built up; it is the method of development, the way the not-yet-able becomes the able. To dismiss imitation as mechanical copying is to miss that in the child it is the creative act by which mind grows.345
The terms of interaction of the two principles, finally, require that the reaction maintain its stimulus, and that this stimulus again repeat the reaction. The one type of reaction, therefore, which an organism must have, is a 'circular' or stimulus-repeating one. We have found it best to name this type of reaction Imitation.Mental Development in the Child and the Race, pp. 487–488
But this is imitation. We have just been giving, as may have been evident, the basis of what is usually called the ' instinct of imitation. ^he instinct to imitate operates by the use of the movements required to do the thing imitated. But unless the child has a sense of what movements will do it, he cannot produce them.Social and Ethical Interpretations in Mental Development, p. 102
But on the same day. May 24, other efforts to bring out imitation failed signally, i.e., with more or less articulate sounds, movements of the lips (Preyer's experiments), and opening and closing of the hands. Ten days later, however, she imitated closing the hand on three different occasions.Mental Development in the Child and the Race, pp. 132–133
Things like this are simply bad habits for which the parents have themselves to thank. The child adapts himself to his treatment, and it is his treatment that his habits reflect. During the second half-year — sooner or later in particular cases — the child is ready to begin to imitate. Imitation is henceforth, for the follow- ing few years, the most characteristic thing about his action.The Story of the Mind, pp. 79–80
IMITATION: A CHAPTER IN THE NATURAL HISTORY OF CONSCIOUSNESS1 IMITATION is a matter of such familiarity to us all that it goes usually unattended to; so much so that professed psychologists have left it largely undiscussed.Fragments in Philosophy and Science, pp. 168–169
The circular reaction is one of Baldwin's most durable contributions: Piaget made it the central mechanism of sensori- motor development, and it remains a standard concept. Baldwin's elevation of imitation to the engine of development anticipated a major modern theme — the human capacity for imitation as foundational to learning and culture — vindicated by findings of neonatal imitation (Meltzoff), the mirror-neuron literature, and social-learning theory, after a long behaviorist interlude that treated imitation with suspicion. The specific architecture is dated, but the insight that imitation and self-reproducing action build the mind is broadly confirmed.