PART V 



CHAPTER XIX 



EVOLUTION AND HEREDITY OF ANIMALS AND 

 PLANTS 



491. Definition of Evolution. Many times in the pre- 

 ceding chapters we have had occasion to point out that 

 similarity of structures suggests relationship of animals and 

 plants, and that all organisms have undergone more or less 

 change in adaptation to the environments in which they live, 

 A remarkable set of examples are the adaptations of roots, 

 stems, leaves, flowers, and fruits described in Chapter VIII. 

 In all these cases the similarity of structure suggests that 

 the adapted organs (roots, stems, etc.) have been derived 

 from the typical forms of these organs as seen in other plants. 

 For example, fleshy roots have developed from plants with 

 ordinary tap-roots, irregular flowers adapted to insects 

 probably descended from regular flowers; and so in all 

 special adaptations of plants close examination suggests 

 the line of modificat on in making special adaptations and 

 points out the relationship. 



As an example on the animal side, it has been shown that 

 the various forms of feet of hoofed mammals appear to have 

 been derived from ancestors with five toes ( 361), and that 

 general similarity in structure of all parts of their bodies points 

 to relationship between all the ungulates. Such facts of 

 similarity which suggest relationship are fundamental to 

 the study of Organic Evolution, by which is meant the theory 

 2o 561 



