HEREDITY. 



No book, purporting to give a review of the influence of 

 experimental evidence since Darwin has had on our knowledge 

 about evolution, would be complete without treating of the 

 mechanism of heredity. Do we know more about the mecha- 

 nism of heredity than Darwin did, and if so, how does our 

 knowledge affect our understanding of evolution? 



The view of Darwin, that heredity is a transmission from 

 parent to off -spring of protoplasmic units, in some way deter- 

 mining the characters of the new individuals is still prevalent 

 in a slightly modified form in very many theories of heredity. 

 Weismann's hypothetical "determinants" are thought to be 

 influenced by the characters of the parents and to "deter- 

 mine" the characters of the offspring ; according to him there 

 exist reciprocal relations between "germplasm" and soma. 



De Vries' pangens, although called by the name Darwin 

 gave to his hypothetical bearers of hereditary characters, are 

 differently conceived. De Vries' idea of pangens is an "intra- 

 cellular pangenesis" and he does not believe in a migration 

 of pangens through the individual or in mysterious relations 

 between the final qualities of an organism, its reactions upon 

 the environment, and its germ-cells. 



From all kinds of experiments on grafting, but especially 

 from the results of Baur and Winkler's work on periclinal 

 chimeras, we clearly perceive, that cells do not modify the 

 inheritable constitution of neighbouring cells, and that there- 

 fore de Vries' conception of the mechanism of heredity is 

 nearer the truth than Darwin's or Weismann's. 



It matters little whether hypothetical determinants are 

 thought to be diffused throughout the cell, or localized in the 



