As to the names and relative order, or rank, of the subdivisions that we shall have occasion to mention, a few words are desirable. The only real fact is the individual animal. A collection of these so similar that they cannot be divided, and which will interbreed, but usually are sterile as to other animals, is termed a _species_. A number of species closely similar are bracketed together as a _genus_ (plural _genera_), and this done, every individual is given a double name, as _Felis leo_ to the lion, the first part of which indicates its genus, and is called its "generic" name, and the second indicates its species, and is called its "specific" name. This "scientific name" is given in Latin (or Latinized Greek) so that it may be unmistakably understood in all parts of the world, for a local name in one language would mean nothing to a student speaking some other language, or perhaps speaking the same language in another country; thus the name "robin" is applied to half a dozen very different birds in separate parts of the English-speaking world, and endless confusion would result were not each animal labeled in a language understood by everybody; and this must be a dead language, so that the significance of the terms applied shall not vary in place or time. Several similar genera may form a _family_; families that agree in essential characteristics are united as _orders_; orders are grouped into _classes_; and finally like classes are assembled into a _phylum_ (Greek, "a leaf": plural _phyla_), which is the largest division except the primary distinction of Protozoa and Metazoa. CHAPTER IV THE HUMBLEST OF ANIMALS--SPONGES