250 CLASSIFICATION OF PLANTS AND ANIMALS 



We have learned something about the various physiological pro- 

 cesses of plants and of animals, and have found them to be in many 

 respects identical. We have found grades of complexity in plants 

 from the one-celled plant to the complicated flowering plants of 

 considerable size and with many organs. So in animal life, from 

 the protozoa upward, there is constant change, the change being 

 usually toward greater complexity of structure and functions. A 

 worm is a higher type of life than a protozoan, because its struc- 

 ture is more complex. A fish is a higher type of animal than a 

 worm, for this same reason, and also because it has an internal 

 skeleton. It is a vertebrate animal. 



We have now learned that animals may be arranged in groups, 

 beginning with very simple one-celled forms and culminating with 

 man himself. These groups are believed by scientists to represent 

 different stages in the complexity of development of life on the 

 earth. Geologists find in the rocks of this earth many fossils, or 

 remains of plants and animals that have long been dead. They 

 teach that the earliest forms of life upon the earth were very 

 simple, and that gradually more and more complex forms ap- 

 peared, as the rocks formed latest in time show the most highly 

 developed forms of plant and animal life. 



Man's Place in Classification. — Man is the only creature that 

 has moral and religious instincts. Although we know that man is 

 separated mentally by a wide gap from all other animals, in our study 

 of biology we must ask where man is to be placed. If we attempt 

 to classify man structurally, we see at once that he must be 

 placed with the vertebrates because he has a vertebral column. 

 Evidently, too, he is a mammal, because the young are nour- 

 ished by milk secreted by the mother, and because his body 

 has at least a partial covering of hair. Finally, man is placed 

 in the highest order of mammals, the primates, because he walks 

 erect and his fore appendages (the arms) are each provided with 

 a hand. 



Development of Man. — Undoubtedly there once lived upon the 

 earth races of men who were much lower in their civilization than 

 the present inhabitants. They were probably nomads, wandering 

 from place to place, feeding upon wild fruits, seeds, roots, and what- 

 ever living things they could kill with their hands. Gradually 



