STAGES IN PLANT EVOLUTION 45 



fossils, and it will be seen that of the seventeen groups, 

 so many as four are known only in the fossil state. 

 This indicates, however, but a part of" their importance, 

 for in nearly every other group arf many families or 

 genera which are only known as fdssils, though there 

 are living representatives of the group as a whole. 



In this table the individual families are not men- 

 tioned, because for the present we need only the main 

 outline of classification to illustrate the principal facts 

 about the course of evolution. As the table is given, 

 the simplest families come first, the succeeding ones 

 gradually increasing in complexity till the last group 

 represents the most advanced type with which we are 

 acquainted, and the one which is the dominant group 

 of the present day. 



This must not be taken as a suggestion that the 

 members of this series have evolved directly one from 

 the other in the order in which they stand in the table. 

 That is indeed far from the case, and the relations 

 between the groups are highly complex. 



It must be remarked here that it is often difficult, 

 even impossible, to decide which are the most highly 

 evolved members of any group of plants. Each indi- 

 vidual of the higher families is a very complicated 

 organism consisting of many parts, each of which has 

 evolved more or less independently of the others in 

 response to some special quality of the surroundings. 

 For instance, one plant may require, and therefore 

 evolve, a very complex and well-developed water-car- 

 riage system while retaining a simple type of flower; 

 another may grow where the water problem does not 

 trouble it, but where it needs to develop special methods 

 for getting its ovules pollinated; and so on, in infinite 

 variety. As a result of this, in almost all plants we have 

 some organs highly evolved and specialized, and others 

 still in a primitive or relatively primitive condition. It 

 is only possible to determine the relative positions ot 

 plants on the scale of development by making an aver- 



