There is a sentence in the short story The Law of Life, "It was the law of all flesh.” What does the word ‘flesh' mean here?

A、Humanskinandthehumanbody.;
B、Thesoftinsidepartofafruitorvegetable.;
C、Thesoftpartofananimal'sbodybetweenthebonesandtheskin.;
D、Allthelivingbeings.