Good Question. This is the way the words are used today, but Aristotle's had only two words for these heirarchical layers, because Biology was not very well developed at this time. He was the first modern biologist. "Genus" and "Species" or their Greek equivalents were thus used to refer to every layer of the a category system. I mentioned this in Footnote #3. WHen studying logic, we are only looking at a relationship between two different categories, so we usually use "Genus" and "Species", no matter where those categories are in some previously developed hierarchy.