individuals of which the species consist are not only related by resemblance of structure, but also by their origin--being supposed to have derived their origin from a parent of original creation; while the other groups have, as far as we know, no other relation than that of similarity of structure. It is obvious that the characters of the various groups might be founded upon peculiarities of any kind. But on this point two methods must be specially distinguished, in one of which the groups are founded upon the sum of all the peculiarities, while in the other they are based upon the structure of single parts or organs. In the former case the system is called natural, in the latter artificial. And while the latter often brings together beings which have perhaps but one or two points of resemblance, and separates others which are closely related, the former associates those which are really and naturally similar. All the groups have special names, so that they may be referred to and spoken of as in the case of common things; the names being composed of Greek or Latin words, so that they may be intelligible to all nations; and as these are dead languages, they will remain good for all time. In mentioning the name of an animal or plant, the name of the genus is always used with that of the species; thus, the name of Chickweed is _Stellaria media_. Because there are mostly several species in a genus; so that if the name of the genus only were used, the species meant would be uncertain; and as there are often species of the same name in different genera, if the name of the species only were used, the genus meant would be doubtful. The classification of animals and plants serves two important purposes: one is that the structural peculiarities and affinities of the groups may be contrasted and a knowledge of their absolute and their differential characters acquired, and for this a natural system is eminently serviceable; the second is that of enabling any animal or plant to be simply distinguished from any other, for which an artificial or analytical system is extremely useful. CHAPTER XIV. OPTICAL PRINCIPLES.