pay nature's debt

[…] and that a younger famished dog should by this man at Mr. Knott's expense be acquired and held in reserve, against the day when the first famished dog should die, and that then again another famished dog should in the same way be procured and held in readiness, against the inevitable hour when the second famished dog should pay nature's debt, and so on indefinitely, there being thus two famished dogs always available, […]