As has been stated in the introductory part of this book, if all the animals that have ever lived on the globe should be represented by a tree those existing on the earth to-day would be indicated by the topmost twigs and leaves, while the extinct forms would be represented by the trunk and main branches. Just as the leaves, twigs, branches, and trunk of the tree have a common origin, viz., the seed that developed into the tree, so all the different species of animals of the present and the past are the trunk, branches, twigs, and leaves of the “tree of life,” and have had a common origin from a primitive protozoan cell (see Diagram of Development, Fig. 18). Therefore all creatures, living and past, have a more or less blood relationship. The Diagram of Development will indicate in a very general way the possible track taken by a man as he evolved,—grew higher and higher as the central, straight trunk of the expanding tree of life,—during the geologic ages; and finally appeared as the inflorescence of the topmost branch of this central trunk. It is seen from this scheme that the tree of life commenced in a primitive cell. Without entering into any discussion of the various theories of evolution and epigenesis, we may say that the primitive protozoan contained potentially all the animal forms (each being a cell or group of cells) that have existed on the globe, just as the fertilized egg contains potentially all the tissues and organs (groups of cells) of the adult man. As the tree of animal life unfolded and expanded—like a germinating seed—from the primitive protozoan, certain of the descendants evolved along the straight and central branch, through the primitive colonial protozoans, on through primitive vermes, and still on through primitive fishes (elasmobranchii), amphibians, reptiles, and on through primitive ornithodelphia (monotremes), and didelphia (marsupials) to a primitive order of monodelphia, viz.: primitive primates. The evolution of man continued through primitive anthropoidea to primitive anthropopithecus. At this point we meet with the common ancestors of the _higher anthropoid apes_ (chimpanzee and gorilla) and _man_. [Illustration: FIG. 18.—Diagram of Development: Portion of the “Tree of Life,” showing approximately the relative places of the great groups of animals. The _Central Trunk_ and _Primary Branches_ represent _Primitive_ (geologic) forms; the _Terminal Twigs_ represent _Modern_ forms.]