382 PROGRESSIVE DEVELOPMENT 



simplest character. The fossils of the most highly developed 

 animal, so far discovered, found in the oldest rock (Paleozoic) 



Flying reptiles, the pterodactyls, were probably 

 the ancestors of our modern birds. 



are those of an animal related to a crab, called the trilobite. As 

 higher layers of rock are reached, fossils of more complex animals 

 are found. These are different, but obviously related to the 

 forms in the strata below. 



According to the present-day evidences, all life at an early period 

 was in the water. The history of the development of life has 

 been formulated from records made in rocks. It is thought that 

 the most primitive forms probably appeared in the warm shallow 

 waters of the flats between tides. First, there appeared plant 

 forms, algae ; later, animal forms, protozoa. There was no life 

 on the land. The rocks and coarse soil were quite bare. Later, 

 countless minute creatures appeared in the water. Their shells 

 made up the great cliffs of chalk. 



After centuries had passed an organism appeared that had an 

 organ for breathing air. It is thought that this form developed 

 from the more primitive types. The first traces of the land ver- 

 tebrates are the footprints of the amphibia. These are preserved 

 in mud molds which have solidified into rock. 



