222 THE PATH OF DEGENERATIVE EVOLUTION 



CHAPTER I 



DO INSTITUTIONS OR ORGANS WHICH HAVE DIS- 

 APPEARED REAPPEAR ? 



SECTION I 

 Disappeared organs 



IN biology we are almost unaware of indisputable 

 examples of the normal reappearance of disappeared 

 organs. 



1. Plants. As the embryonic development of 

 plants is usually direct, it is impossible to decide 

 whether an organ which forms a component part 

 of the embryological history represents an ancestral 

 organ. However, in a few rare cases, artificial 

 selection causes an actual reversion of evolution. 



Typical geraniums possess two whorls of five 

 stamens, as, for instance, in Geranium. In Erodium 

 there is only one cycle of five. In Pelargonium one 

 cycle of five is complete ; the other is represented 

 by two stamens and three filaments which have 

 lost their anthers. But in certain varieties with 

 very large flowers the two complete cycles re- 

 appear, five stamens having long, and five short, 

 filaments. In this case there is no doubt as to 

 the reappearance of the three stamens lost in 

 typical Pelargoniums. 



