222 PHYSICAL BASIS OF CIVILIZATION 



distinction between preservers of heredity and 

 media of modification. 



The next higher step in reproduction is by gem- 

 mation. In this process a portion of the cell of a 

 unicellular organism is extruded, or a cell or some 

 cells of a multicellular organism are extruded, and 

 this extruded part then begins the independent 

 growth of a new individual. 



At this stage the differentiation between the 

 external and central substance has evidently gone 

 far enough to justify calling the central substance 

 reproductive material. Some of this carrier of 

 heredity is then extruded to build the new individ- 

 ual. The central substance in this stage must 

 have been less subject to disturbance by the environ- 

 ment than in the one previously mentioned. The 

 purifying process, the segregation of like from unlike, 

 alluded to heretofore, has had more time to achieve 

 its tendencies. 



To give proper emphasis to this line of reasoning, 

 it should be borne in mind that a part of the repro- 

 ductive substance passes on from generation to 

 generation, practically forever. The whole organism 

 except this reproductive substance is composed of 

 somatic cells, which die out completely in each 

 generation, leaving nothing from which the bodies 

 of the next generation could be grown. These 

 latter have to be and are built by that portion of 

 the reproductive material which is passed over 

 by each generation to the next. If some reader 



