I was going to propose that orcs might be evolved from amphibians or reptiles, but being cold-blooded doesn't quite pan out for them.

Maybe insects. The type you find when you turn over a rock. Like ants or rollipollis.

I propose that all Elves, Drow, Dryads, Nyphes and other anthromorphs of the natural variety are all desendents of a Base Line that we will call the Elf-Kin.

I would go further and say that all the humanoids likely came from the same common ancestor.

However, I'm going to play around with Nebra's idea, but it's a much more interesting idea that they evolved independently.

On that idea, I'd say the if Dwarves and dogs are closely related, then so are cats and Elves. They are both graceful species that are as much at home in trees as they are on ground (Just look at how leaf jumped from that tree in the earlier strip). Both are often depicted as hunters that can travel silently.

If by halflings, you mean Hobbits, I'd say they're similar enough to humans that they share a common ancestor.

Pixies are easy. They are related, and not too distantly, to birds. Specifically, Magpies. Enough said.

The one's that fly don't get much bigger than commercial aeroplanes, though size varies by species.