[Mamzelle]

They cannot "interbreed" because of size--a Great Dane pup would kill a papillion mother. This brings a certain surface appeal to your argument, but the gap between type and species is a vast one. I could also show you an angora goat, tiny little thing with long snow-white ringlets, and a big homely Nubian with hardly any hair at all, much older breeds than the dogs you describe.

Your "leap" is still a huge one. Dogs are still dogs--goats are still goats.

It'd be easier to work with insects--shorter life cycle, simpler creature.

[Modernman]

They cannot "interbreed" because of size--a Great Dane pup would kill a papillion mother. This brings a certain surface appeal to your argument, but the gap between type and species is a vast one

The point is, with dogs we are observing speciation in action. Dogs at either end of the spectrum cannot inter-breed, whereas dogs that are closer together physically (say, a rottweiler and a standard poodle, imagine what that would look like) can. If we keep doing what we're doing with dogs, down the road the descendants of various breeds will certainly be considered different species.

Remember, we've only been messing with dogs for a few thousand years. Where do you think we'll be a million years from now?

[Ichneumon]

They cannot "interbreed" because of size--a Great Dane pup would kill a papillion mother.

Yes. Exactly.

This brings a certain surface appeal to your argument,

Translation: "Uh oh, that's undermining my point, I'd better try to hand-wave it away as just "a certain surface appeal", without actually refuting it..."

but the gap between type and species is a vast one.

Feel free to define it, then.

Your "leap" is still a huge one. Dogs are still dogs--goats are still goats.

Humans are still primates... Still apes too, for that matter.