These secondary processes and thoughts can eventually turn into an adaptation or provide a fitness advantage to humans. Just because something is a secondary trait or byproduct of an adaptation does not mean it has no use. In 1982, Gould and Vrba introduced the term "exaptation" for characteristics that enhance fitness in their present role but were not built for that role by natural selection. Exaptations may be divided into two subcategories: pre-adaptations and spandrels. Spandrels are characteristics that did not originate by the direct action of natural selection and that were later co-opted for a current use. Gould saw the term to be optimally suited for evolutionary biology for "the concept of a nonadaptive architectural by-product of definite and necessary form – a structure of particular size and shape that then becomes available for later and secondary utility". Criticism of the term Gould and Lewontin's proposal generated a large literature of critique, which Gould characterised as being grounded in two ways. First, a terminological claim was offered that the "spandrels" of Basilica di San Marco were not spandrels at all, but rather were pendentives. Gould responded, "The term spandrel may be extended from its particular architectural use for two-dimensional byproducts to the generality of 'spaces left over', a definition that properly includes the San Marco pendentives." Other critics, such as Daniel Dennett, further claimed (in Darwin's Dangerous Idea and elsewhere) that these pendentives are not merely architectural by-products as Gould and Lewontin supposed. Dennett argues that alternatives to pendentives, such as corbels or squinches, would have served equally well from an architectural standpoint, but pendentives were deliberately selected due to their aesthetic value. Critics such as H. Allen Orr argued that Lewontin and Gould's oversight in this regard illustrates their underestimation of the pervasiveness of adaptations found in nature. Response to criticism Gould responded that critics ignore that later selective value is a separate issue from origination as necessary consequences of structure; he summarised his use of the term 'spandrel' in 1997: "Evolutionary biology needs such an explicit term for features arising as byproducts, rather than adaptations, whatever their subsequent exaptive utility ... Causes of historical origin must always be separated from current utilities; their conflation has seriously hampered the evolutionary analysis of form in the history of life." Gould cites the masculinized genitalia of female hyenas and the brooding chamber of some snails as examples of evolutionary spandrels.