That is so as regards the functional products of human evolution.

The hand was produced out of the animal forelimb with the primary end of grasping the things we materially need, but as a by-product the hand has developed the function of making and playing the piano and the violin, and that secondary functional by-product of the hand we account, even as measured by the rough test of money, more precious, however less materially necessary, than its primary function.

It is, however, only in rare and gifted natures that transformed sexual energy becomes of supreme value for its own sake without ever attaining the normal physical outlet.

For the most part the by-product accompanies the product, throughout, thus adding a secondary, yet peculiarly sacred and specially human, object of marriage to its primary animal object.

This may be termed the spiritual object of marriage.
By the term "spiritual" we are not to understand any mysterious and supernatural qualities.