Let us extend this reasoning to categories of organisms of higher orders than species. We can associate together groups of species in the same way that we associate together the individuals of the same species. There are certain morphological characters which are common to all the species in the category, but there are also differences between specific group and specific group, and these differences may be regarded as variations from the generic morphological type. All the Cats, for instance, have certain characters in common: fully retractile claws, a certain kind of dentition, certain cranial characters, and so on. We postulate a feline type of structure, and we then regard the characters displayed by the cat, lion, tiger, leopard, etc., as deviations from this feline morphological type. Thus we establish the Family Felidæ. But again we find that the Felidæ together with the Canidæ, and many other species of animals, also display common characters, dental and osteological chiefly, and we express this resemblance by assembling all these families in one Order, the Carnivora. The Carnivores, however, are only one large group of Quadrupeds: there are many others, such as the Rodents, Ungulates, Cetacea, etc., and all of these possess common characters. In all of them the integument is provided with hairs, or other similarly developed structures; all breathe by means of a diaphragm; in all, the young are nourished by suckling the mammæ of the mother; and all develop on a placenta. We therefore group them all in the Class Mammalia. Now the Mammals possess an internal skeleton of which the most fundamental part is an axial rod--the notochord--developing to form a vertebral column; and this notochordal skeleton is also possessed by the Birds, Reptiles, Amphibia, and Fishes. There are also some smaller groups in which the notochord is present but does not develop to form segmented vertebræ. Including these, we are able to form a large category of animals--the Chordata--and this phylum is sharply distinguished from all other cognate groups. All animals and plants may be classified in a similar way. Insects, Spiders, and Crustacea, for instance, are all animals in which the body is jointed, each joint or segment being typically provided with a pair of jointed appendages or limbs. Because of this similarity of fundamental structure we include all these animals, with some others, in one phylum, the Arthropoda. So also with the rest of the animal kingdom, and similar methods may be extended to the classification of the plants. A few small groups in each of the kingdoms are difficult to classify, but it has been possible to arrange most living organisms in a small number of sub-kingdoms or phyla, and even to attempt to trace relationships between these various categories.