A. Three-fold Development. 157 



called Paleozoic Time, and extends from the beginning 

 of the Cambrian formation to the close of the Carbon- 

 iferous period. The third or Mesozoic Time begins with 

 the Triassic period and ends with the Cretaceous. Last 

 comes Cenozoic Time, reaching to the present. The 

 Mesozoic Time, with which we will now spend a few 

 moments, has been namied the Reptilian age. It includes 

 and is divided into three periods, the Triassic, Jurassic 

 and Cretaceous. So far in our study of the development 

 of life we have found a gradual but sure elevation in 

 speciallization and character as indicated by the periods. 

 Land plants and animals have appeared, the plants first 

 and then the animals. As fast as suitable conditions 

 were developed in the physical inorganic world, then a 

 higher type of life has made its appearance. We shall 

 find this to be the case all through the geological ages of 

 the world. A change in the geographical form of land 

 and sea has produced a different climate. This has 

 brought about a different plant life, followed by a cor- 

 responding change in the animal kingdom. Each has 

 been the effect of previous changes and the cause of fol- 

 lowing ones. The Mesozoic Time brought into existence 

 a wonderful change in terrestrial life. The first mam- 

 mals, both marine and land, appeared. The first Birds, 

 the first of the common bony fishes, such as our rivers 

 and lakes are now filled with, are found in deposits of 

 this time. The first Palm trees grew on the land then 

 above the waters. All of our common, well known trees, 

 both forest and fruit, first grew and blossomed in this 

 age. The lack of carbonic acid in the atmosphere com- 

 pelled the plants to increase their leaf surface, and this 

 same lack of nutriment gave us the slower growing and 

 more solid wooded tree trunks, without which we could 

 not have had our lumber. The animal life was largely 

 aquatic or amphibian, and the seas swarmed with mon- 

 strous saurians, which were all carniverous and cold 

 blooded. In the latter part of this time, the Cretaceous 

 period, were laid down the formations of Northwest 

 Iowa, on which the glacial clay or loess was afterwards 

 spread over in the socalled Ice age. All the life of the 

 Mesozoic Time was moving upward and shadowing forth 

 the higher forms of the succeeding, the Tertiary age. 



