PLANT EVOLUTION 237 



covered to great depths by rock, soil or water, and areas that 

 were once covered with water may now be found at great eleva- 

 tions. Nature has left readable records of these great changes 

 and among the most important of these records are the fossil 

 remains of extinct plants and animals. A study of these fossil 

 remains shows gradual changes in the forms of plant life on 

 the earth. These changes have very generally been from lower to 

 higher types, but there is an abundance of evidence that lower 

 types may be derived from higher types (see -page 273). Fur- 

 thermore, the predominating forms of life have not always been 

 the same. No doubt there must have been a period when the 

 algae were predominant. This was followed by the age of 

 mosses and then the age of ferns. More recently the coniferous 

 plants were most abundant, and this period was followed by the 

 development of the flowering plants of the present. 



Geographical Record. The present distribution of plants 

 over the face of the earth involves the study of ecology and plant 

 geography to which we have already referred. The most im- 

 portant factors influencing the distribution of plants are water, 

 temperature, soil, light, wind, mountain ranges, deserts, rivers, 

 lakes, seas, animals and man ; all of which have been discussed 

 to a greater or less extent in the preceding chapters. The study 

 of the relationship of plants to these environmental agencies has 

 led to the grouping of plants that grow under the same conditions 

 into societies, such as hydrophytic, mesophytic, xerophytic, al- 

 pine, prairie and numerous others of more or less importance. 

 The grouping of plants into these societies has very little rela- 

 tion to the classification of plants in accordance with the natural 

 system which will be discussed in Part III of this book. 



Theories of Evolution. It is very easy to point out simi- 



