ARE ACQUIRED CHARACTERS HEREDITARY? 415 



plasm." According to this view as brought out in another place 

 (p. 201), there is an unbroken continuity from generation to generation 

 of the germ plasm. Germ cells are thought of as remaining entirely 

 undifferentiated for any somatic function and as therefore capable of 

 starting at the beginning to develop a new individual. The germ cell 

 is supposed to be "set apart at an early period in a given individual; 

 it takes no part in the formation of the individual's body, but remains 

 a slumbering mass of potentialities which must bide its time to awaken 

 into expression in a subsequent generation." 



Physiologists object to this idea that the germ cells are so dis- 

 tinctly different from body cells and that they are so insulated, as it 

 were, from the soma as to be immune to any changes that may affec. 

 the latter. Two kinds of data are offered in opposition to this con 

 cept. A few observers, notably Professor C. M. Child, have described 

 cases in which somatic cells, that already had become differentiated 

 as primitive muscle cells, lost their differentiation and returned to a 

 germinal condition. If this kind of thing were general, and it is 

 probably not, germ cells might conceivably be produced from func- 

 tioning soma cells and might therefore furnish a mechanism for the 

 transmission of the effects of use and disuse. It should be empha- 

 sized, however, that, among animals at least, there is extremely little 

 evidence in support of the idea that differentiated body cells give rise 

 to germ cells. 



Among plants, however, a different situation prevails. In the 

 Begonia, for example, any part of a plant if cut off is capable of pro- 

 ducing a whole new plant. Even a purely vegetative organ like a leaf, 

 if cut off and partially buried in soil, will bud off a new plant which 

 will produce flowers with perfectly typical germ cells. We have to 

 admit, in this case, either that leaf tissues contain undifferentiated germ 

 cells or that somatic tissues give rise to germ cells. The first alterna- 

 tive is in harmony with the germ-plasm hypothesis, the second is 

 the preferred view of the opponents of this hypothesis. 



Among animals, as for example annelid worms, it is quite common 

 to find the germ cells aggregated in a few segments of the body. If a 

 part of the body in which there are no recognizable germ cells be cut 

 off, it will, under proper conditions, regenerate the lost parts and 

 become a complete worm with functional germ cells. The same 

 alternative explanations that were offered for the Begonia case apply 

 equally well here. Numerous other cases of the same sort are web 1 

 known to all zoologists. To the advocate of the " germ-plasm " theory 



