[matz1]

>Of course we would never confuse another species for a human

Yes, this is what I mean. There is seemingly no organism/species that I would confuse whether to classify it as human or not human but I'm pretty sure there are species/organism albeit is rare that I would be in hard time categorizing it as cat or not cat.

Another example is human male and human female, albeit is rare but there is some human population that I would have a hard time deciding which category to put.

[mannykannot]

It is highly likely that there once were intermediate species. When examined in sufficient detail, the line between ancestor and descendant species blurs.

[Jweb_Guru]

Even if you might confuse a domestic house cat for some other creature, I'm pretty sure another domestic house cat would not, which is the relevant point of comparison here :)