Chapter V 



EVOLUTIONARY DEVELOPMENT AND 

 PAST HISTORY 



In Chapter II we gave a brief general account of each of the main 

 classes of the plant kingdom now living, mentioning that the sequence 

 used was probably indicative of evolutionary history at least in broad 

 outline. The classes treated were usually those most important as 

 components of vegetation at the present time, regardless of their 

 significance in the past. Those omitted even included some that 

 had been vastly important in earlier ages but had subsequently 

 become extinct or nearly so. 



Evolution is a continuous process, that started with the earliest 

 forms of life and still goes on abundantly today. And even as the 

 numerical representation and importance of a particular group of 

 organisms may go down as well as up, so may evolution manifest 

 itself in simplification or decline as well as advance. This is parti- 

 cularly evident in many parasitic forms of both plants and animals. 

 Nevertheless the general trend is towards advancement in complexity 

 if not always in size, economy in the use of material being also 

 important ; with these tendencies and the need for adaptation to the 

 environment and other circumstances constantly in mind, we can 

 place the forms known to us in a sequence that seems most likely 

 to be the actual one of their own evolution. This will be done 

 briefly below for those groups that are important as fossils, regard- 

 less of the significance of relatives living today, but with close 

 reference to these in order to link relationships in the mind's eye. 



It is necessary here to give an outline account of the geological 

 time-sequence. In this, four major eras are defined, the last three 

 of which are divided into several periods or epochs each. It is now 

 believed that the earth had its beginnings about 4,500,000,000 years 

 ago. The latest measurements indicate the age of the oldest known 

 rocks to be about 3,300,000,000 years, and the era from that time 

 to about 550,000,000 years ago constitutes the pre-Cambrian. This 

 era has often been divided into two — the ' older ' Archaeozoic (of 

 metamorphic and igneous rocks) and the ' younger ' Proterozoic (of 



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