Why is it impossible to judge the species of an organism by one criterion?

Because one and the same trait can be found in completely different species of animals. For example, let’s look at the appearance of a living organism. There are two types of black mice – absolute copies of each other. But you cannot get a common offspring from them, since they have a different genotype. That is, they are two different types. And by their external similarity, we could attribute them to one. The same applies to the spruce bough and the pine bough – similar, like twin brothers, they are different types of bough and live separately. Known doubles among mushrooms – edible and their poisonous copies.