contingent fact needs diverse class member to be verified

We are aware that flopping is necesary for flying is a specific to bird, because we had airplanes now, and they fly without flopping.

We don’t have a proper definition for general intelligence, nor do we know any of the human nature is really necessary to the general intelligence that human poses, because we have yet to find another form of general intelligence apart from human.

It is impossible to verify if any fact we assume about a class is actually contingent to a specific member, if the whole class consists of a sole member.