It's also just an extremely impractical definition. Say you have two animals and you want to know if they are the same species. How are you going to find out wh

It's also just an extremely impractical definition. Say you have two animals and you want to know if they are the same species. How are you going to find out whether they can or cannot mate (and produce fertile offspring, even)? What if they are the same species but just aren't that interested in each other?