Triangles can take on an infinite number of variations in
dimension and proportion, for example, and still remain perfect
triangles.
But they can't gain or lose a corner.
Not so with organisms. Tetrapods can lose all four limbs and stay tetrapods.
Why? Because there are descended from tetrapods. There is an essence, but it
is descent, not any merely three-dimensional quality. There is no way to
ignore time in this question. Tree-thinking. Four-dimensional. :-)