In studying animals we are always haunted by the fear that the analogy from him to man may not hold; that some element essential to the development of the human mind may not be in the animal at all.

Even in such a question as the localization of the functions of the brain described later on, where the analogy is one of comparative anatomy and only secondarily of psychology, the monkey presents analogies with man which dogs do not.
But in the study of children we may be always sure that a normal child has in him the promise of a normal man.
3.

Again, in the study of the child's mind we have the added advantage of a corresponding simplicity on the bodily side; we are able to take account of the physiological processes at a time when they are relatively simple--that is, before the nervous system has grown to maturity.

For example, psychology used to hold that we have a "speech faculty," an inborn mental endowment which is incapable of further analysis; but support for the position is wanting when we turn to the brain of the infant.

Not only do we fail to find the series of centres now known to be the "speech zone," but even those of them which we do find have not yet taken up this function, either alone or together.