of which shall not be recognized as sufficiently specialized to be called either plants or animals, but simply organisms. But this suggestion seems to meet with little practical favor from students of systematic biology.

For a basis, therefore, of any study of the evolution of life, an acquaintanceship with the life and structure of the simplest organisms is a necessity. As the authors have already tried in another book ("Animal Life") to present a simple account of this life together with an account of certain less simple or slightly complex organisms (Figs. 22-26) whose physiology and structure reveal successive stages in organic complexity and specialization, and as the space in this book is limited, the authors must refer their present readers to chapters I, II, and III of "Animal Life" for an account of the life of the simplest and slightly complex organisms.

FIG. 20. Parama

cium aurelia. At each end there is a contractile vacuole, and in the center is one of the nuclei. (After Verworn.)

The differentiation and growing complexity of the body of those many-celled animals which differ from and are, we may say, beyond and higher than the simple manycelled forms, are by no means always along the same line (Figs. 27-37). It is familiar knowledge that animals can be classified or grouped into a number of great divisions called branches or phyla. For example, the starfishes, sea urchins, sea cucumbers, etc., constitute one phylum, the Echinodermata; the crustaceans, insects, spiders, etc., constitute another phylum, the Arthropoda, and all the animals with a backbone or with a notochord constitute another, the Chordata. of these phyla there is a fundamental or type structure (Fig. 27). All of the Echinodermata, for example, are built on the radiate plan. They recall the starfish with its five or more arms radiating from a central disk. The Arthropods are all animals with a body composed fundamentally of a series of successive segments, some or all of these segments bearing pairs of jointed appendages; and so on. We need not pursue

Now for each

FIG. 22.-Gonium pectorale, a colonial protozoön: A, seen from above; B, seen from the side. (After Stein.)

of fishes, amphibians, reptiles,

birds, and mammals.

In all the subdivisions of the main groups there are also to be recognized differentiated and divergent lesser lines of descent, and within these still lesser ones. While, as already noted, the main divisions of the animal kingdom are called phyla and the divisions of the phyla, classes, the subdivisions of the classes are usually called orders. The next subdivision is that into families, each in, turn being a cluster of genera. The genera are composed of species and the species finally of sub-species, varieties, and individuals. Each one of these

A

FIG. 23.-Pandorina sp., a colonial protozoon. (Highly magnified.)

FIG. 24.-A fresh-water polyp, Hydra vulgaris: A, in extended condition and in contracted condition; B, cross section of body, showing the two layers of cells which make up the body wall.

names refers primarily to a special line or mode of differentiation and at the same time refers to the fact that the members of each of these differentiated groups are genetically related to each other, that is, related by blood, by actual ancestral descent. All these differentiated groups indicate diverging lines of evolution, some of them short, and but slightly divergent from the main

line from which

they arise; others, on the contrary, long, important, and widely divergent.

The traditional tree which is drawn to explain animal classification illustrates at the same time the two fundamental facts upon which this classification is based, namely, differentiation of structure, and corresponding divergence of descent. All the branches of this genealogical tree lead back, as they do in a real tree, to its trunk, and the trunk of this tree springs from the simplest of the many-celled animals, namely, from those primitive forms which resemble in essential characters animals like the

FIG. 26. One of the sim

plest sponges, Calcolynthus primigenius. A part of the outer wall is cut away to show the inside. (After Haeckel.)

simpler polyps. Indeed it seems certain that this tree trunk can be traced farther back; that it must spring in the beginning from forms essentially like the lowest organisms that we know to-day, namely, single, simple cells living independently. From the Amoeba to Man; that is the history of descent, or ascent if one prefers. The course has been a continuous one, both in point of time and in point of gradual transformation.

FIG. 27.-Diagram showing fundamental structure of types of several animal phyla: 1, sea anemone; 2, starfish; 3, worm; 4, centipede; 5, clam; 6, honeybee; 7, salamander. In each figure the central nervous system is indicated by the black lines. (After Haeckel.)

But great periods of this time are shut away from us without. record of their duration, and long series of the gradually changing forms are lost to us without hope of discovery. And yet in its large outlines we know the history of all this time and the character of all these graded series.