CHAPTER XX. 



EVOLUTION, HOMOPLASY, ANALOGY, AND HOMOLOGY. 



In the preceding pages the Higher Plants have been described with 

 only occasional allusion to organisms which, being simpler in their 

 structure and mode of life, are placed lower in the scale of the Vege- 

 table Kingdom. In taking the Highest Plants first, a similar sequence 

 has been followed to that which has ruled in the development of the 

 Science itself, as its history plainly shows. It was natural that the 

 earlier observers should direct their attention to those plants which 

 were most obvious, and seemed to them to be the most useful and 

 common. Consequently the structure of the science was founded at 

 first upon observation of Flowering Plants. Later, when botanists 

 began to examine smaller and simpler organisms, the tendency was 

 to interpret them in terms of the Higher Plants. But this procedure 

 is a practical inversion of the course which the real history of the 

 Vegetable Kingdom is now believed to have taken. Comparison of 

 the simpler organisms with the more complex has led with ever 

 increasing certainty to the conclusion that the latter were derived by 

 Descent from the former. The simpler organisms are now held to 

 represent types such as appeared earlier upon the Earth. The more 

 complex types, on the other hand, are held to have been derived 

 from ancestors simpler than themselves, and to have been later in 

 their origin. This being the view generally entertained, it will be 

 well for us to break off for the moment from the discussion of the 

 higher forms, and to set back to those lowest in the scale. By doing 

 so, and then by proceeding gradually to those which are more com- 

 plex, it will be possible to trace in general those lines along which 

 Evolution appears to have progressed in producing the Higher Forms 

 of Vegetation. 



While we thus consciously adopt the general conclusion that the 



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