The bond is simply inheritance, that cause which alone, as far as we positively know, produces organisms quite like each other, or, as we see in the case of varieties, nearly alike.
Lokarri hau herentzia besterik ez da, kausa honek, jakin dakigunez, beren artean erabat antzekoak, edo, aldakien kasuan ikusten dugun moduan, ia berdinak diren organismoak sortzen dituelarik.