  In a bicategory of spans (an example of a 'generic bicategory') the
factorization of a span (s,t) as the span (s,1) followed by (1,t) satisfies a
simple universal property with respect to all factorizations in terms of the
generic bicategory structure. Here we show that this universal property can in
fact be used to characterize bicategories of spans. This characterization of
spans is very different from the others in that it does not mention any
adjointness conditions within the bicategory.
