22 2 Heredity and Eugenics 



this capacity, therefore, for the production of pigmentation 

 is not something which is conditioned by any particular 

 thing in the cell, but it represents a capacity common to all 

 living substances. 



In what way is the constitution of the germ cell 

 modified so that the organism shows in subsequent gen- 

 erations a permanent change in its coloration? It has 

 been pointed out to me by Professor Morgan that in these 

 experiments the behavior in the first generation is difficult 

 of interpretation. In these experiments the male ceUs 

 or the female cells, and sometimes both, have been sub- 

 jected to conditions of experiment at a susceptible stage. 

 Eggs are most susceptible immediately before and during 

 maturation, although what connection this has to the 

 maturation process is not known. 



Morgan has raised the question, why do individuals, de- 

 veloped from eggs which have been subjected to conditions of 

 experiment and fertilized with normal sperm, not give a sub- 

 sequent hybrid behavior ? No hybrid splitting has ever been 

 found in any of my experiments, or in those of MacDougal 

 or Gager. The resulting modification reproduces itself true 

 to type, and does not give subsequent splittings suggestive 

 of the combination of different factors or unit-characters. 

 If there are unit-characters, it is logical to expect that in 

 experiments of this kind the experiment would modify 

 the unit-character in the germ plasm, and that this modified 

 unit-character would then behave, when crossed with its 

 normal homologue, exactly as hybrids do in other cultures. 

 The total lack of this behavior in my experiments, and those 

 of MacDougal, Gager, and others, might be considered 

 good evidence that there are no such things as unit- 

 characters, nor in the germ cells any potentiality capable 



