[Illustration: The geological history of the horse. (After Mathews, in the American Museum of Natural History.) Ask your teacher to explain this diagram.] Increasing Complexity of Structure and of Habits in Plants and Animals.--In our study of biology so far we have attempted to get some notion of the various factors which act upon living things. We have seen how plants and animals interact upon each other. We have learned something about the various physiological processes of plants and animals, and have found them to be in many respects identical. We have found grades of complexity in plants from the one-celled plant, bacterium or pleurococcus, to the complicated flowering plants of considerable size and with many organs. So in animal life, from the Protozoa upward, there is constant change, and the change is toward greater complexity of structure and functions. An insect is a higher type of life than a protozoan, because its structure is more complex and it can perform its work with more ease and accuracy. A fish is a higher type of animal than the insect for these same reasons, and also for another. The fish has an internal skeleton which forms a pointed column of bones on the _dorsal_ side (the back) of the animal. It is a vertebrate animal. [Illustration: The evolutionary tree. Modified from Galloway. Copy this diagram in your notebook. Explain it as well as you can.] The Doctrine of Evolution.--We have now learned that animal forms may be arranged so as to begin with very simple one-celled forms and culminate with a group which contains man himself. This arrangement is called the _evolutionary series_. Evolution means change, and these groups are believed by scientists to represent stages in complexity of development of life on the earth. Geology teaches that millions of years ago, life upon the earth was very simple, and that gradually more and more complex forms of life appeared, as the rocks formed latest in time show the most highly developed forms of animal life. The great English scientist, Charles Darwin, from this and other evidence, explained the theory of evolution. This is the belief that simple forms of life on the earth slowly and gradually gave rise to those more complex and that thus ultimately the most complex forms came into existence. The Number of Animal Species.--Over 500,000 species of animals are known to exist to-day, as the following table shows. Protozoa 8,000 Arachnids 16,000 Sponges 2,500 Crustaceans 16,000 Coelenterates 4,500 Mollusks 61,000 Echinoderms 4,000 Fishes 13,000 Flat-worms 5,000 Amphibians 1,400 Roundworms 1,500 Reptiles 3,500 Annelids 4,000 Birds 13,000 Insects 360,000 Mammals 3,500 Myriapods 2,000 ------- Total 518,900