A "species" is defined as a a group that can naturally breed and produce viable offspring (that can itself breed).
Mastiffs and Chijuaja(sp?) cannot naturally breed. Ergo, although they had a common ancestor just a few hundred years ago, they no are no longer the same species.
And no, artificial insemination does not count.
If so, lions and tigers would be the same specifies (the resulting offspring is called a Liger, which do exist, albeit they are generally sterile like a mule, which is itself an inter-species cross ).
In fact, HUMANS and CHIMPS can be bred together via artificial insemination. The Chinese did this, impregnating female chimps with human sperm. Fortunately, the resulting creature was destroyed before term (or so the Chinese claim).
It doesn't? Gee, a lot of horse farms will be interested to know that their horses ain't horses.
Since you credit the Chinese story of inter-primate breeding, should I believe you when you define two breeds of dogs as two species?
"In vitro" is different from "in vivo" from "in the wild." (Seems the Romans were very civilized, no Latin for "in the wild.")
Do you have a source for this?
The only limitation here is physical, same as with some humans, Pygmies and Zulus, comes to mind, but no one would claim that they both are not the same species. A dog in heat is a dog in heat, regardless of size, shape or demeanor. The attempt would be made by them in spite of size difference. And if successful would produce viable reproductive offspring....
Saruman did it first...
"We had many of these half-orcs to deal with at Helm's Deep."