Another thing that partly determines and modifies this order is the child's own unwearied practice in forming consonant-sounds. He hears his own voice now better than he did at an earlier period when he was forming vowels only. He most easily retains and repeats, among the infinitely manifold consonants that are produced by loud expiration, those which have been distinctly heard by him. This is owing to the association of the motor and the acoustic memory-image in the brain. These are the most frequent in his speech. Not until later does the mechanical difficulty of articulation exert an influence, and this comes in at the learning of the compound sounds. Hence there can not be any chronological order of succession of sounds that holds good universally in the language of the child, because each language has a different order in regard to the frequency of appearance of the sounds; but heredity can have no influence here, because every child of average gifts, though it may hear from its birth a language unknown to its ancestors, if it hears no other, yet learns to speak this language perfectly. What is hereditary is the great plasticity of the entire apparatus of speech, the voice, and with it a number of sounds that are not acquired, as _m_. An essential reason for the defective formation of sounds in children born deaf is the fact that they do not hear their own voice. This defect may also be hereditary. The treatise of F. Schultze contains, besides, many good remarks upon the _technique_ of the language of the child, but, as they are of inferior psychogenetic interest, they need not be particularly mentioned here. Others of them are only partially confirmed by the observations, as is shown by a comparison with what follows.