CHAPTER X 

 EVOLUTION OF THE PLANT KINGDOM 



The doctrine of evolution states that all forms of life, living and extinct, 

 have been derived from preexisting forms by a process of gradual change. 

 This principle of "descent with modification," supported by an over- 

 whelming mass of evidence, has been fully established as a fundamental 

 axiom of biology. The method by which evolution has taken place, how- 

 ever, is imperfectly understood and considerable uncertainty exists as to 

 the relative importance of the various factors involved. 



These factors are of two kinds : primary or causative and secondary or 

 directive. Causative factors give rise to heritable variations, which are 

 the raw^ materials of evolution. These are built up into new species under 

 the influence of directive factors, which determine the course of evolution. 

 Heritable variations arise both from new combinations of genes in fer- 

 tilization and by mutation, a process involving changes in the chromo- 

 somes of reproductive cells, the causes of which are largely unknown. 

 The addition or loss of one or more chromosomes by irregularities in 

 meiosis may cause changes affecting several or many characters simulta- 

 neously. Much more common and of greater importance are changes 

 involving individual genes, these giving rise to innumerable small varia- 

 tions that supply most of the raw^ material upon which the directive 

 factors of evolution operate. Thus evolution is mainly dependent upon 

 the appearance of mutations, especially gene mutations. 



The general trend of evolution is toward greater fitness to the conditions 

 of existence. Some of the variations arising by recombination of genes 

 or by mutation are adaptive, while others are not. By natural selection, 

 favorable variations having survival value are preserved and accumulated 

 through successive generations, thus bringing about greater adaptation to 

 the environment. Natural selection determines which individuals among 

 a diversified population shall survive in the "struggle for existence." It 

 is generally regarded as the most important directive factor yet 

 discovered. 



Theories dealing with the causes of evolution are concerned chiefly with 

 the origin of species. One of the great problems of morphology is the 

 determination of the origin and phylogenetic development of the larger 

 plant groups. The fossil record demonstrates that groups once dominant 

 on the earth have been replaced by others more advanced, but it seldom 



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