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 
