Again, in the body of each individual there are parts which are intended for a certain purpose, as the organs of nutrition for the preservation of the individual, the organs of generation for the preservation of the species, the hands and eyes for administering to certain wants, as to food, etc.; there are also parts which, in themselves, are not intended for any purpose, but are mere accessories and adjuncts to the constitution of the other parts. The peculiar constitution of the organs, indispensable for the conservation of their particular forms and for the performance of their primary functions, produces, whilst it serves its special purpose, according to the nature of the substance, other things, such as the hair and the complexion of the body. Being mere accessories, they are not formed according to a fixed rule; some are altogether absent in many individuals; and vary considerably in others. This is not the case with the organs of the body. You never find that the liver of one person is ten times larger than that of another person, but you may find a person without a beard, or without hair on certain parts of his body, or with a beard ten times longer than that of another man. Instances of this phenomenon, viz., great variation as regards hair and colour, are not rare. The same differences occur in the constitution of the Universe. Some species exist as an integral part of the whole system; these are constant and follow a fixed law; though they vary as far as their nature permits, this variation is insignificant in quantity and quality. Other species do not serve any purpose; they are the mere result of the general nature of transient things, as, e.g., the various insects which are generated in dunghills, the animals generated in rotten fruit, or in fetid liquids, and worms generated in the intestines, etc. In short, everything devoid of the power of generation belongs to this class. You will, therefore, find that these things do not follow a fixed law, although their entire absence is just as impossible as the absence of different complexions and of different kinds of hair amongst human beings. In man there are substances the individual existence of which is permanent, and there are other substances which are only constant in the species not in the individuals, as, e.g., the four humours. The same is the case in the Universe; there are substances which are constant in individuals, such as the fifth element, which is constant in all its formations, and other substances which are constant in the species, as, e.g., the four elements and all that is composed of them.