first, it was essential to be able to apprehend in the depths of the body the relations that link superficial organs to those whose existence and hidden forms perform the essential functions; thus Storr proposes mammals according to the formation of their hoofs; the reason being that this is linked to methods of locomotion and to the animal's possibilities of movement; now, these methods of locomotion can be correlated in turn with the form of alimentation and the different organs of the digestive system.