When did "species" become the word for both the singular and plural? [duplicate]

According to the following sources, the origin of the word comes from the Latin 'species' and the term has been used with biological reference both as singular and a plural since the 17th century:

Species:

• The noun species, referring especially to a group of organisms sharing common characteristics, can be either singular (e.g., that species is purple) or plural (e.g., these species are yellow). This is the convention in scientific writing, and it is usually followed elsewhere.

• The word does share a Latin origin with the singular noun specie, but species and specie have diverged in meaning over the centuries and are now unrelated in all their main uses. Specie now refers primarily to coin money, and it appears in the phrase in specie, which means in coin, in kind, or (in law) as specified.

Source:http://grammarist.com/usage/species/

Etymology, Species:

• late 14c. as a classification in logic, from Latin species "a particular sort, kind, or type" (opposed to genus), originally "a sight, look, view, appearance," hence also "a spectacle; mental appearance, idea, notion; a look; a pretext; a resemblance; a show or display," typically in passive senses; in Late Latin, "a special case;" related to specere "to look at, to see, behold," from PIE *spek- (see scope (n.1)). From 1550s as "appearance, outward form;" 1560s as "distinct class (of something) based on common characteristics." Biological sense is from c.1600. Endangered species first attested 1964.

Source:http://www.etymonline.com/index.php?term=species&allowed_in_frame=0