154 MORPHOLOGY, EVOLUTION, AND CLASSIFICATION 



belief in organic evolution, or the theory of descent, chiefly through 

 the work of Charles Darwin, whose famous book, The Origin of 

 Species, appeared in 1859. The theories of organic evolution 

 hold that all the existing species of animals and plants have 

 been derived or evolved through the geological ages from the 

 simplest forms of life in the beginning. These theories also 

 hold that the kinds now on the earth are subject to change, and 

 that very many of them are in process of developing new species. 

 There are varying opinions as to the causes which bring about 

 changes in species, and there are several schools of evolutionists 

 whose theories are the subject of constant discussion and inves- 

 tigation. 1 But all botanists and zoologists believe in the main 

 principles of organic evolution ; and the theory is the framework 

 of biology. Indeed, the theory of organic evolution is as impor- 

 tant to biology as the atomic theory is to chemistry and as the 

 doctrine of the conservation of energy is to physics. 



191. An outline of the classification of plants. There will 

 be given for reference at this point a classified arrangement of 

 the most important of the larger groups of plants. It is quite 

 impossible to develop a classification very far in the compass 

 of this book, but this outline will serve to indicate the field 

 covered in the succeeding chapters. 2 The thallophytes are 

 especially difficult to classify, for the groups are not as clearly 

 understood as those of the higher plants, and there are complex 

 relationships, especially between the algse and the fungi. The 

 relationships among the green algse are also not well under- 

 stood, and ths arrangement presented here is largely one of 

 convenience in the present state of our knowledge of this puz- 

 zling assemblage of forms. Classifications are, of course, subject 

 to constant modification, as groups receive more careful study, 

 and authors differ widely in their systems. 



1 See Chapter xxxix, Variation, Mutation, and Origin of Species. 



2 For the most recent and detailed classification of plants the reader is 

 referred to Engler, Syllabus der Pflanzenfamilien, 1907, or to Engler and 

 Prantl, Die Naturlichen Pflanzenfamilien. 



