A scientist researching salamanders has obtained two different specimens. Under what conditions would he classify them as belonging to different species?

This is the condition under which he would classify them as belonging to different species - the two salamanders cannot mate to produce a fertile offspring. If these two lizards belonged to the same species, they would normally be able to breed and give birth to a fertile new "child." However, given that they are not of the same species, they cannot do such a thing, which is why this scientist classified them that way.