>    The definition of species is related to genetics, not to behaviour.

I disagree.

By human intervention, you can cause all kinds of matings to occur that 
would never, ever happen in nature. That does /not/ mean that all 
animals are of the same species.

>    The offspring must be fertile. Ligers and tigons aren't.

Apparently it was long thought that ligers and tiglons were infertile, 
but apparently they have in a few rare cares produced long-lived offspring.

But in any case, yes, I agree that the offspring must be fertile.

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