CHAPTER XVI. 

 HOMOLOGOUS PARTS AND RUDIMENTARY ORGANS. 



What is meant by homology? 



Darwin expresses it as follows: 



„The members of thesame class,independently of their 

 „habit of Hfe resemble each other in the general plan 

 „of their organisation. This resemblance is often 

 ..expressed by the term ..unity of type" or by sa5dng 

 ..that the several parts and organs in the different 

 ..species of the class are homologous" 



Hence homologous parts are parts occupying corres- 

 ponding positions in the general groundplan. 



Therefore one is tempted to consider the corollae of 

 two flowers to be homologous, however different, 

 while bracts, however corolla-like, are certainly not 

 homologous with the petals which form the corolla, 

 but only analoga of these. 



Now the determination of what is homologous and 

 what is analogous, consequently depends on the cer- 

 tainty with which we can determine the position 

 of a part in the general groundplan. and as the name 

 ..general" groundplan already indicates some un- 

 certainty in this respect, it is no wonder that one 

 meets in the literature with endless discussions as to 

 which parts are homologous and which analogous. It 

 was to call attention to this fact, that I said above, that 

 we are tempted to consider aU corollae to be homolo- 



