How was mendel able to derive postulates concerning the behavior of unit factors during gamete formation, when he couldn't directly observe them?

A postulate is just more or less an assumption of a fact or truth. I think Mendel was able to postulate concerning the behavior of unit factors during gamete formation because he studied this area well and base on his readings he was able to imagine what might be happening during this time.

The hawk receives 0.1% energy from the caterpillar although the caterpillar gets eaten by another bird smaller than the hawk first