1. Language cannot be arbitrary, either in the syntactic processes or in the forms of individual words observed in different languages. Because language is not arbitrary, there can be an inherent predisposition to learn some one language out of a wide range of particular languages (selection from a range of possible lexicons and possible syntaxes) in very much the same way as a bird is innately programmed to learn the particular song of its species (if it is exposed to that song) or a cat is innately prepared, through the structuring of its visual system at birth, to perceive certain visual patternings in its environment (if it is exposed to patternings of that kind). The process by which a particular language is selected and learnt is completely analogous to the plasticity in a critical period or imprinting found in many species of animals: that is, a readiness during a critical learning period for neurological structure to be modified and permanently 'set' by the characteristics of the environment encountered by the young creature.