The first use of the voice was probably that indicated by our frogs and
toads--it serves as a sex-call. That is the meaning of the trumpeting
with which frogs herald the spring, and it is often only in the males
that the voice is well developed. But if we look forward, past
Amphibians altogether, we find the voice becoming a maternal call
helping to secure the safety of the young--a use very obvious when young
birds squat motionless at the sound of the parent's danger-note. Later
on, probably, the voice became an infantile call, as when the unhatched
crocodile pipes from within the deeply buried egg, signalling to the
mother that it is time to be unearthed. Higher still the voice expresses
emotion, as in the song of birds, often outside the limits of the
breeding time. Later still, particular sounds become words, signifying
particular things or feelings, such as "food," "danger," "home,"
"anger," and "joy." Finally words become a medium of social intercourse
and as symbols help to make it possible for man to reason.
§ 2