This matter has been ably and thoroughly discussed by Dr. De Groot,[198] who shows convincingly that "since very ancient times pines and cypresses have played a prominent part as producers of timber for coffins, and that this was the case because these trees, being believed to be imbued with great vitality, might counteract the putrefaction of the mortal remains." The same cause that made such timber valuable for coffins made it valuable for graveyards. The superstition is connected with the ancient Chinese philosophic doctrine of the _Yang_ and the _Yin_--the complementary forces and qualities which pervade all nature, such as male and female, light and dark, warmth and cold, activity and passivity, positive and negative, life and death. It was supposed that all evergreens must have a greater store of the _yang_ element (life, vitality) than other trees, because they retain their foliage through the winter; and of evergreen trees those prized most by the Chinese for their life-giving qualities were and are the fir and the cypress.[199] Therefore by planting these trees in their graveyards and in the courtyards of their ancestral temples the Chinese supposed they would endow their ancestors (apparently both their dead bodies and their living spirits) with a never-failing preservative against decay and dissolution. The result of this on themselves--the living descendants of the dead--must be, it was thought, a constant flow of happiness and good fortune. It will be remembered that ancestor-worship is not merely regarded as a method of showing love and reverence for the dead but is believed to induce the ancestral spirits to protect and watch over the family and to bestow on its members long life, many children and general prosperity. The more abundant the vitality (if one may speak of the vitality of a ghost) that can be imparted to the ancestral spirits, the better able will they be (so goes the theory) to exert themselves on behalf of the fortunes of their posterity; and the best way to impart vitality (that is, the _yang_ element) to the spirits is to surround their coffins and their ancestral tablets with as many _yang_-supplying agencies as possible. The original theory of the matter is probably extinct at Weihaiwei if not everywhere else; trees are planted and protected in the family temples and graveyards for no known reason except that it is the traditional custom to do so[200]: yet it is noteworthy that the cypress is still the favourite tree in the grounds of the ancestral temples, that the fir is still considered one of the best trees to plant in a graveyard, and that the pedigree-scrolls preserved among the archives of every family are often decorated with the painting of an evergreen tree.[201]