We have already learned that resemblances between animals indicate (usually) relationship, and that classifying animals is simply expressing or indicating these relationships. When we group several species together to form a genus we indicate that these species are closely related. And similarly a family is a group of related genera. =Order.=--There are other groups[6] higher or more comprehensive than families, but the principle on which they are constituted is exactly the same as that already explained. Thus a number of related families are grouped together to form an _order_. All the fowl-like birds, including the families of pheasants, turkeys, grouse and quail, all obviously related, constitute the order of gallinaceous birds called _Gallinæ_. The families of vultures, hawks and owls constitute the order of birds of prey, the _Raptores_, and the families of the thrushes, wrens, warblers, sparrows, black-birds, and many others constitute the great order of perching birds (including all the singing birds) called the _Passeres_. =Class and branch.=--But it is evident that all of these orders, together with the other bird orders, ought to be combined into a great group, which shall include all the birds, as distinguished from all other animals, as the fishes, insects, etc. Such a group of related orders is called a _class_. The class of birds is named _Aves_. There is a class of fishes, _Pisces_, and one of frogs and salamanders, _Batrachia_, one of snakes and lizards called _Reptilia_, and one of the quadrupeds which give milk to their young called _Mammalia_. Each of these classes is composed of several orders, each of which includes several families and so on down. But these five classes of Pisces, Batrachia, Reptilia, Aves and Mammals agree in being composed of animals which have a backbone or a backbone-like structure, while there are many other animals which do not have a backbone, such as the insects, the starfishes, etc. Hence these five backboned classes may be brought together into a higher group called a _branch_ or _phylum_. They compose the branch of backboned animals, the branch _Vertebrata_; all the animals like the starfishes, sea-urchins and sea-lilies which have the parts of their body arranged in a radiate manner compose the branch _Echinodermata_; all the animals like the insects and spiders and centipedes and crabs and crayfishes which have the body composed of a series of segments or rings and have legs or appendages each composed of a series of joints or segments make up the branch _Arthropoda_. And so might be enumerated all the great branches or principal groups into which the animal kingdom is divided.