Bearers of Hereditary Characters 205 



some weights are heaviest there is rigidity in inheritance, 

 but where the physiological weights are more prominent 

 there is plasticity in inheritance. Speaking in terms of 

 ontogeny, all embryo tissues are plastic and mature 

 tissues are rigid. Applying this thought to phylogeny 

 it may be assumed that young unit characters are plastic 

 and may be modified by physiological conditions; while 

 old unit characters are fixed, rigid, and determined by 

 chromosomes, against which the influence of physiological 

 conditions is feeble and avails little or nothing. 



The second theory contains more physiological 

 complexities, sketching a picture which is harder to 

 visuaHze as a whole but is more exact in its details. 

 As previously mentioned, the physiologist would like 

 to believe that the protoplasts of each variety of plant 

 show a certain specificity in their chemical make-up. 

 The physiologist finds that the cytoplasm is the seat of 

 the most important physiological activities, and con- 

 sequently expects to find that the cytoplasm is also in 

 some way the bearer of hereditary characters. He does 

 not readily agree with the cytologist that hereditary 

 characters are limited entirely to the chromosomes. He 

 cannot, however, controvert the great bulk of data, 

 collected by cytologists and Mendelians, indicating that 

 chromosomes are the bearers of hereditary characters. 

 He must reconcile his views with these undeniable facts. 

 Perhaps he may be able to do this in the following 

 manner. 



In the first place how must he visualize the so- 

 called "determiners"? Physiologically they seem to 

 behave more like enzymes than anything else that we 

 can think of. If, then, these enzymes are transmitted 



