For every species of plant and animal there exists a type specimen. This
specimen is neither a perfect example nor an average example of the species,
but simply a typical member of it. When a species, subspecies or variety is
first described, a dried herbarium specimen is selected as the type specimen.
It is impossible to foresee how a taxon might be understood in the future. For
example two species may be physically very similar but have completely
different genetic systems that dont allow them to interbreed. In their
outward appearance they may only differ in very small differences of their
anthers, pollen grains, leaf hairiness etc. If we had to rely upon a description
it might be that the originator of the name didnt discuss these features.
However, because the taxonomist nominated a Type specimen this can be
examined for the novel characters.