It has frequently been pointed out that “da” is among the first of
the voluntary sounds produced by a baby. Why is this? The reason is
probably that the sounds which are first produced are those which
require the least effort; these are the guttural sounds, produced
by the mouth organs, produced further back, near the throat, and
the dental sounds, in the production of which the teeth (or gums)
coöperate with the tongue. These nerves and muscles, controlling these
portions of the anatomy, are also those connected with the primative
sensorimotor activity, so that the first sounds are produced in
connection with some physical motion--such as pointing. The nervous
impulses which are utilized in pointing also pass over and assist in
producing the first primative sounds, which as we have seen are the
dental and guttural.
Why, then, does the baby say “da” instead of “ga” when it points?
The answer is to be found in the considerations which we have just
applied. The act of pointing sets into motion one of the extremities
of the body. The nervous process, one of whose divisions goes into
this extremity, can naturally reach by one of its other divisions more
easily the muscles of the frontal part of the mouth, belonging to
the periphery (surface) of the body, than the muscles of the throat,
belonging to a different, an internal, system of muscular activity. Try
yourself to accompany a pointing movement by a dental or a guttural
sound. The latter seems more natural. (See Max Meyer: “The Fundamentals
of Human Behavior.”)