morgan's explanation for why humans live in groups was largely based on the notion that all humans have an inherent "natural valuation" of genealogical ties (an unexamined assumption that would remain at the heart of kinship studies for another century, see below), and therefore also an inherent desire to construct social groups around these ties. even so, morgan found that members of a society who are "not" close genealogical relatives may nevertheless use what he called "kinship terms" (which he considered to be originally based on genealogical ties). this fact was already evident in his use of the term "affinity" within his concept of the "system of kinship". the most lasting of morgan's contributions was his discovery of the difference between descriptive and classificatory kinship terms, which situated broad kinship classes on the basis of imputing abstract social patterns of relationships having little or no overall relation to genetic closeness but instead cognition about kinship, social distinctions as they affect linguistic usages in kinship terminology, and strongly relate, if only by approximation, to patterns of marriage.