6o THE STORY OF PLANT LIFE 



this by means of embryology. For the history of 

 the individual repeats the history of the race. This 

 is the principle of recapitulation which has largely 

 been deduced primarily from a study of extinct or 

 fossil types. 



A study of the early stages of plants also shows the 

 real significance of certain organs, which becomes less 

 marked in the adult. When such organs obscure 

 the affinity of a plant, the early seedling stages may 

 give a clue to the origin, as in the case of the early 

 leaves of the Furze, or Barberry. 



Some plants, however, owing to various causes, 

 tend to exhibit more than one type of structure in the 

 case of one or more organs, or dimorphism. The 

 leaves of some plants are polymorphic. Many differ- 

 ent groups of plants exhibit outwardly similar struc- 

 tures, which in reality are not homologous, and the 

 plants are really derived from widely different ances- 

 tors. Such organs are of multiple origin. It is 

 possible to recognise that an organ of a species in one 

 group is homologous with that of an organ of a species 

 in another group. In the same group allied species 

 possess organs that may be regarded as homologous. 

 But in the former case it may not be due to deriva- 

 tion of the types from a common ancestor. 



If the evidence of palaeontology, or in this special 

 case of palaeobotany, be taken into consideration, it is 

 found, as in the case of morphology and embryology, 

 to bear witness to evolution in the past. 



Present day forms are in fact only the last stages 



