HEREDITY AND EVOLUTION 97 



division by which the continmty of organization 

 is maintained seems clearly enough to be genetic- 

 ally irreversible. It flows forward from germ- 

 cell to germ-cell in endless succession. It is peri- ~ 

 odically diverted from the germ-stream to form 

 the bodies? of successive generations of individ- 

 uals. These are made of the same stuff as the 

 stream from which they flow. In each genera- 

 tion the germinal stuff runs through the same] 

 series of transformations; hence that reappear- 

 ance of the same traits in successive generations 

 that we call heredity. 



This conclusion loses nothing of its force by 

 reason of the fact that in ^exual reproduction 

 or regeneration the whole body may be repro- 

 duced from a fragment, from a small group of 

 cells, or even from a single cell, of the soma. 

 These cells, too, have arisen by division in un- 

 broken descent from the germ-cell; they, too, 

 have been made from the same original stuff ; and 

 they, too, hand on by division to their descendants 

 the specific tradition of their hneage. It is true 

 that these cells and the germ-cells aUke grow by 

 the intussusception of matter from vdthout, that 

 the cell-substance is built 'from, and its activities 

 modified and controlled by, materials that have 

 been elaborated by other cells. But the whole 

 force of the evidence goes to show that their fun- 

 damental basis is determined by genetic contin- 

 uity with that of their predecessors, that some- 

 thing is handed on by division which holds 



