Sign languages have two strikingly different kinds of morphological structure: sequential and simultaneous. The simultaneous morphology of two unrelated sign languages, American and Israeli Sign Language, is very similar and is largely inflectional, while what little sequential morphology we have found differs significantly and is derivational. We show that at least two pervasive types of inflectional morphology, verb agreement and classifier constructions, are iconically grounded in spatiotemporal cognition, while the sequential patterns can be traced to normal historical development. We attribute the paucity of sequential morphology in sign languages to their youth. This research both brings sign languages much closer to spoken languages in their morphological structure and shows how the medium of communication contributes to the structure of languages.* Si l'on pouvait inventer une langue dont les dictions eussent leur signification naturelle, de sorte que tous les hommes entendissent la pensée des autres à la seule prononciation sans en avoir appris la signification, comme ils entendent que l'on se rejoueit lorsque l'on rit, et que l'on est triste quand on pleure, cette langue serait la meilleure de toutes les possibles; car elle ferait la mesme impression sur tous les auditeurs, que feraient les pensées de l'esprit si elles se pouvaient immédiatement communiquer entre les hommes comme entre les Anges. (Mersenne, Harmonie universelle, 1636) [If one could invent a language whose expressions had their natural signification, so that all men could understand the thought of others by pronunciation alone without having learned its signification, as they understand that one is happy when one laughs, and that one is sad when one cries, this language would be the best of all possible: for it would make the same impression on all hearers as would the thoughts of the spirit if they could be communicated immediately between men as between the angels.] [our translation] If humans could communicate by telepathy, there would be no need for a phonological component, at least for the purposes of communication; and the same extends to the use of language generally. (Noam Chomsky, The minimalist program, 1995:221)