CHAPTER X A . 



GENESIS, HEREDITY, AND VARIATION 

 CONCLUDED. 



97a. SINCE the foregoing four chapters were written, 

 thirty-four years ago, the topics with which they deal have 

 been widely discussed and many views propounded. Ancient 

 hypotheses have been abandoned, and other hypotheses, re 

 ferring tacitly or avowedly to the cell-doctrine, have been 

 set forth. Before proceeding it will be well to describe the 

 chief among these. 



Most if not all of them proceed on the assumption, shown 

 in 66 to be needful, that the structural characters of organ 

 isms are determined by the special natures of units which are 

 intermediate between the chemical units and the morphologi 

 cal units between the invisible molecules of proteid-sub- 

 stances and the visible tissue-components called cells. 



Four years after the first edition of this volume was 

 published, appeared Mr. Darwin s work, The Variation of 

 Animals and Plants under Domestication; and in this he set 

 forth his doctrine of Fangcnesis. Referring to the doctrine 

 of physiological units which the preceding chapters work out, 

 he at first expressed a doubt whether his own was or was not 

 the same, but finally concluded that it was different. He was 

 right in so concluding. Throughout my argument the im 

 plication everywhere is that the physiological units are all 

 of one kind; whereas Mr. Darwin regards his component 

 units, or &quot; gemmules,&quot; as being of innumerable unlike kinds. 

 He supposes that every cell of every tissue gives off gemmules 

 35G 



