THE EVOLUTION OF PLANTS 1 25 



98. 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 V and VIII. 

 By this means we may learn something of the relationship 

 of different groups having Hving representatives, but it 

 chiefly serves to throw light on the method of evolution. 

 The course of evolution is best ascertained by the observa- 

 tion and comparison of plant structures. 



99. Sources of Evidence. — There are five main sources 

 of evidence as to the course of evolution: 



1. Comparative life histories of living forms. 



2. Comparative anatomy of living forms. 



3. Geographical distribution. 



4. Structure of fossil forms. 



5. Geological succession of fossil forms. 



Studies along these five different Knes have resulted 

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

 from the various sources k\\ points to the same broad 

 conclusions. Conflict or contradication is in most cases 

 the result of insufl&cient evidence from one or more sources. 



100. Evidence from Life Histories. — In the study of 

 the life history {ontogeny) of any higher sporophyte, 

 we find that vegetative (sterile) tissues develop first. On 

 the basis of this fact it has been inferred by some in- 

 vestigators^ that all reproductive organs (stamens, carpels, 

 sporophylls) arose by a modification of vegetative organs. 

 Other facts, however, as set forth on pages 126-129, have 

 lead to the directly opposite conclusion. 



'See Bower, F. O. "The origin of a land flora." Macmillaa and Co. 

 Ltd., London, 1908. 



