I have a few questions about your hominid scale thing. What's with Homo sentiens and Homo sapiens? It seems a bit improbable to lump all fantasy races into two single species too. I personally say gnomes are "Homo gnomus", elves are "Homo alfus", goblins are "Goblinus kobalos", giants are "Gigantohomo gigantus", and tinies are "Homo lilliputianus".
You know, you could just have posted this among the comments for he chart in question. :P
But to answer your question: I've divided the human races into *three species* (Homo sentiens, Homo intelligens, and Homo sapiens), based on two things: their most recent common ancestor, and their ability to produce fertile offspring with one another (which are usually the main criteria in biology). All subspecies (races) within their respective species can, at least theoretically, produce fertile offspring with one another, provided their genitals are compatible (size difference usually being the main obstacle. Between the species, however, fertile offsprings are rare, and usually the exception that proves the rule. So elves and manshes will generally produce fertile offspring, while elves and orcs will mostly produce sterile offspring.
Furthermore, the races' ancestry is often evident in shared characteristics within the species themselves, such as skin colour ranges, bone structure, and other physiological differences that are greater between the different species that within them, so a clear line can be drawn. For example: Homo intelligens possess sloped foreheads, greater hairgrowth (whether local or universal), and greater sensitivity to sunlight; Homo intelligens possess greater muscle density, larger lower canines (tusks), and more pronounced brow ridges; Homo sapiens feature greater stamina, smoother skin, and better climate adaptability.
So overall, it's usually rather easy to pinpoint which species an individual belongs to even from a distance: the differences between orcs and manshes are quite obvious. Elves, manshes and even hobs, on the other hands, may be harder to differentiate without closer inspection: from a distances, hobs may look like simply infant elves and manshes.
Nonetheless, despite their differences, the species all look similar enough to each other to be easily identified as members of a single genus (Homo), wherein they also share a common ancestor.
I'm sorry, I know I could've. I didn't think you'd answer for some reason. Sometimes I act without thinking, and I apologize. Also, that makes sense. Though I feel like giants would warrant their own species rank, with the cyclops being considered a subspecies, unless it's more of a mutation, but that's just my opinion.
Thank you! ^^