Geographic variation and allometry shape head morphology more than diet in the Mexican black-bellied garter snake (Thamnophis melanogaster).

Compositionality is widely regarded as a central guiding principle in semantic theory. In particular, the “rule-to-rule” conception has deep roots in the philosophy of language and remains a standard assumption in the dominant tradition of linguistics (see Montague, ; Partee, ). Rule-to-rule compositionality begins with the thought that complex expressions are derived from the more basic expressions by applying a sequence of syntactic formation rules. Each syntactic rule for deriving a complex expression corresponds to a rule for specifying the meaning of the derived expression in terms of the meanings of the more basic expressions. The principle of compositionality is a high-level methodological hypothesis in linguistics (see Dever, ; Partee, ). It places constraints on the shape of semantic theories, and time and again, data that initially appear resistant to compositional treatment have been successfully analyzed within this framework. Such analyses frequently yield novel predictions. In addition to its role as a methodological hypothesis, the principle is also motivated by the fact that communication in language is productive and systematic. Language users can produce and understand indefinitely many sentences on the basis of their understanding of a limited number of expressions. This ability is explained if the language user has mastered the rules for specifying the derived sentences in terms of the basic expressions and the corresponding rules for deriving the meanings of the derived sentences in terms of the meanings of the basic expressions. Thus, theories are often rejected or modified on account of failing to deliver a compositional analysis of the target phenomenon.