means simply what is visible. As things are distinguished by their visible forms, it has come to mean kind or class. As drugs and condiments at one time formed the most important articles of merchandise, they were called species—still retained in the French épices, and English spices. Again, as bank-notes represent money, money itself is called specie, the thing represented.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894