In respect to primitive works we may distinguish four steps in the acquisition of aesthetic features of form, three of which are normal, the fourth abnormal: First, we have that in which functional characters alone are considered, any element of beauty, whether due to the artist's hand or to the accidents of material, construction, or model, being purely adventitious; second, that in which the necessary features of the utensil appear to have experienced the supervision of taste, edges being rounded, curves refined, and symmetry perfected; third, that in which the functionally perfect object, just described, undergoes further variations of contour, adding to variety and unity, thus enhancing beauty without interfering with serviceability; and, fourth, that in which, under abnormal influences, beauty is sought at the sacrifice of functional and constructive perfection.

What is the underlying theme of the passage?