570 STRUCTURE AND LIFE HISTORIES 



490. Methods of Study. In the solution of this prob- 

 lem two methods of attack may be employed: (i) That 

 of observation and comparison of structure, followed by 

 classification, and inference; (2) that of experiment. The 

 use of experiment is indicated in Chapters XXXIII and 

 XXXV. By this means we may learn something of the 

 relationship of different groups having living representa- 

 tives; but it chiefly serves to throw light on the method of 

 evolution. The course of evolution is best ascertained by 

 the observation and comparison of plant structures. 



491. Sources of Evidence. There are four main sources 

 of evidence as to the course of evolution: 



1. Comparative anatomy of living forms. 



2. Comparative life histories of living forms. 



3. Structure of fossil' forms. 



4. Geological succession of fossil forms. 



Studies along these four different lines have resulted 

 in some conflict of evidence, but on the whole the evidence 

 from the various sources all points to the same broad 

 conclusions. Conflict or contradication is in most cases 

 the result of insufficient evidence from one or more sources. 



492. Evidence from Comparative Anatomy. Compara- 

 tive study of structure has led to the conclusion that, 

 in its broadest aspects, the course of plant evolution has 

 been from the simple to the complex; that such simple 

 organisms as Pleurococcus, and other green algae, preceded 

 more complex forms like the liverworts; that Bryophytes 

 appeared before ferns, and they in turn before the modern 

 Gymnosperms and Angiosperms. 



A difficulty of accepting this conclusion as final is the 

 possibility that, at certain points, the course of evolution 

 may have been retrograde. For example it is generally 



