The adjacent diagram shows a structure (represented by a red dot) built on a set of five distinct elements (represented by blue dots); a corresponding structure could be built out of any five elements.

Two finite sets are in bijection whenever they have the same cardinality (number of elements); thus by definition the corresponding species sets are also in bijection, and the (finite) cardinality of depends only on the cardinality of A. In particular, the exponential generating series