|Microtus californicus californicus|
But a species is meant to be an actual thing, right? Certainly, when the word was coined by Linnaeus (in the biological sense; it has older meanings, too) in the 18th century, that was his intent. A hundred years later, Darwin elaborated on the term, realising, as earlier evolutionary theorists had, but Linnaeus hadn't, that species do slowly change into new forms over time. Today, the most common understanding, outside of scientific circles, is that two animals belong to different species if they can't interbreed to produce fertile offspring.