Halliday & Matthiessen (1999: 613):
The elements of protolanguage are “signs” (that is, content/expression pairs); they are thus formally identical with the semiotic resources of higher mammals (primates and cetaceans) — but with one important difference: the signs of other species become codified as the form of communication among adults, whereas those of children are transitional to a system of a different kind, and hence do not stabilise into a settled pattern but are constantly shifting on both semiotic planes.