987. The nomenclature of plants, collectively and individually. The whole vegetable kingdom is divided into classes, orders, genera, species, and varieties. A class is distinguished by some character which is common to many plants; an order or a tribe is distinguished by having some character limited to a few plants belonging to a class; a still more limited coincidence constitutes a genus; and each individual of a genus, which continues unchanged when raised from seed, is called a species. A variety is formed by an accidental deviation from the specific character, and easily returns by seed to the particular species from which it arose.