Plants and animals can be classed in groups which are contained within larger groups. This classification is not fortuitous like that of pebbles on a beach, nor arbitrary like the grouping of stars in imaginary constellations. The members of each group of plants and animals are placed together because they resemble each other, and the groups in turn are characterized by resemblance. The resemblance between organisms, or between groups of organisms, is due to their relationship (genetic affinity); the difference between them is due to descent with divergence and modification from ancestors shared in common. A natural classification, such as this, provides a family tree of relationship and reflects the course of evolution. In a group classified in accordance with a natural system of classification it is possible to show the affinities and derivations of the various sub-groups in the form of a diagram or evolutionary tree.

The following example is a simple classification based on similarities and differences of body plans is given of the Chordata, which include the vertebrate based on examples of the main groups and explanations of the body structures by which they are classified.

• an internal skeletal rod called a notochord,
• a tubular nervous system running down the back, and
• a series of gill-pouches on each side of the throat.

The Chordata may be divided into those animals without skulls which are called the Acrania, and those with skulls which are called the Craniata. The former include the Cephalochordata (Branchio-stoma), Hemichordata (Balanoglossus), and Urochordata (Tunicates). The latter can be divided into those without jaws, the Agnatha, and those with jaws, the Gnathostomata. The former include the Cyclostomata (lampreys) and some extinct groups. The latter can be divided into those with fins, the Fishes or Pisces, and those with hands and feet ending in fingers and toes, the Tetrapoda.

As an example of a well-defined group, the Pisces may be taken and classified into their chief subdivisions, as follows.

PLACODERMI, a diverse assemblage of extinct fishes, representing a number of early experiments in fish structure. Among them some had jaws in series with the gill-supports much like primitive sharks, but in others they seem to have been simpler. Some of them had paired fins, but these might be in any number from one to seven, and had not developed any of the structural characters which were to make fish fins so efficient either for balance or for locomotion.

CHONDRICHTHYES, fishes with the skeleton made of cartilage (gristle).

SELACHII, sharks and rays.

HOLOCEPHALI, rabbit fishes and their extinct ancestors.

OSTEICHTHYES, fishes with the skeleton largely of bone.

CROSSOPTERYGII, bony fishes in which the paired fins have a central axis, and primitively there are two dorsal fins. The heavy scales of primitive forms were of the kind known as cosmoid. In some, the nostrils communicate with the mouth.

RHIPIDISTIA, mainly piscivorous, among which some had limbs adaptable to crawling, lungs and nostrils adaptable to air-breathing, and which gave rise to the land-vertebrates or Tetra-poda. They also gave rise to:

COELACANTHINI, the Coelacanth fishes.

DIPNOI, which specialised in a crushing dentition and survive in Australia, Africa and South America as the lung fishes.

ACTINOPTERYGII, bony fishes in which the paired fins are fan- shaped, and primitively there is only one dorsal fin. The heavy scales of primitive forms were of the type known as ganoid.

PALAEOPTERYGII, mainly extinct fishes, primitively with heterocercal tails and with the maxilla and preopercular bones as broad cheek-bones. They became very diverse in the Mesozoic era and some of the characters of the next group appeared singly in different species, but not in combination. The survivors are the sturgeons, degenerate in the loss of most of the bone from the internal skeleton, and the specialised bichirs (Poiy-pterus), with paddle-like pectoral fins.

NEOPTERYGII, including most of the present-day bony fishes, The tail-fin has an abbreviated axis; the maxillary bone is freed from the cheek-plates and moves as the main bone of the upper jaw; the preoperculum is boomerang-shaped, accommodating itself to the jaw-suspension apparatus beneath.

HOLOSTEI, the more archaic members of the Neopterygii, with several bones comprising the lower jaw, no supra-occipital bone in the skull and a series of valves counteracting back-pressure of blood as it leaves the heart. Amia and Lepidosteus survive.

TELEOSTEI, the modern branch of the Neopterygii, including nearly all familiar bony fishes. The lower jaw is simplified, a supraoccipital bone is present and the valves at the exit of the heart are reduced.