Animals and Plants of Past and Their Records 615 



sification was made more accurate and complete by incorporating the 

 data contributed by paleobotany. Many large groups of plants of the 

 past, although they have disappeared completely, have thrown much 

 light on the relationships of living plants through their fossil records. 

 Other large groups of plants which were originally dominant have dimin- 

 ished in numbers and importance until they are represented by a limited 

 number of types today. However, they too have contributed to a more 

 accurate classification of present-day plants. 



A study of the fossil remains of ancient plants also reveals certain 

 climatic conditions which prevailed at the time such plants existed. In 

 other words, the presence of large numbers of plants at a certain period 

 precludes a certain type of climate in order that such plants might fluor- 

 ish. Such studies reveal luxuriant vegetation in regions which are at 

 present more or less devoid of that type of plant. In general, the climate 

 throughout a great part of geologic time is thought to have been much 

 more uniform than at present. It is thought to have been quite mild, 

 somewhat like present tropical climates, and with an abundance of 

 moisture. 



A study of paleontology also reveals certain geographic conditions of 

 the past. Regions of the world now united originally may have been 

 widely separated by barriers. Regions once connected are now widely 

 separated. For example, mountains may have arisen or large land areas 

 may have been submerged beneath the water. It is thought that much 

 of the land area of today at some time in the past may have been below 

 the surface of the sea. This is concluded by the type of fossils found in 

 the earth's strata. Such natural phenomena as floods, glaciers, volcanic 

 eruptions, and earthquakes have aflfected plant distribution in the past. 

 A change in the quantity and quality of the atmosphere, water, food, or 

 soil in the past undoubtedly influenced the distribution of plants. The 

 greater and more extensive the changes in this connection, the greater 

 the effects on plants. All of these factors have in the past been quite 

 influential in determining plant growth and distribution. 



A study of the fossil records of plants reveals that there has been a 

 development from the simple to the complex and that the more com- 

 plex flowering plants appear late in geologic history. In other words, 

 throughout geologic time there has been a continued succession of plants. 

 With each era and period, more complex and highly evolved plants be- 

 come dominant, only to be superseded later by newer and more compli- 

 cated groups. An accurate study of the plants of the past and present 

 enables us to reconstruct much of the history of the plant world (Fig. 

 320). 



