CHx\PTER X\ 



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 Pangenesis. Eef erring to the doctrine' 

 of ph3'siological 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 " gemmules," as being of innumerable unlike kinds. 

 He supposes that every cell of every tissue gives off gemmules 

 356 



