[quote=@stone]
[@Idea]

Though, I suppose they might not be species, considering in most settings there are half-elves, half-orcs, etc. That means that they can reproduce and stuff, which fills the species definition.
[/quote]

[i]Technically[/i] if you wanna get scientific about it, [b]if[/b] Elves, Orcs, and Humans etc. are all separate species, if they were to interbreed their offspring should be sterile, unable to reproduce, like mules for example. Assuming this is the case, given that there are some fairly dramatic physical differences between, say, Orcs and Humans, the chances of an Orc-Human hybrid having actual physical deformities, or developmental differences are extremely high and their lifespans will most likely be shorter on average. 

BUT assuming any/all humanoid species can produce offspring that share their parents’ traits but are otherwise physically sound, would mean that [i]technically[/i] they all exist within the same species, and their genetic differences are simply the result of some groups developing different genetic traits over time that eventually become the standard for that group. Think Great Danes vs. Shitzus, if we didn’t know all dog breeds are descended from a common ancestor we would probably think those two dogs are entirely different species. Technically a Great Dane and a Shitzu can reproduce viable offspring, making them the same species.