PROTOPLASM 39 



the fundamental organization. Students of heredity — geneticists 

 — speak of the genetic structure of an organism as contrasted 

 with its outward appearance. 



Irritability and Adaptation 



In every stage of development the substances making up the 

 protoplasmic organization of an individual are constantly react- 

 ing with each other and with those of the environment.* These 

 activities are maintained in a delicately adjusted equilibrium 

 which changes and readjusts itself with each stimulus from with- 

 out. It is this unstable equilibrium, this sensitiveness, which is 

 meant by the expression irritability of protoplasm. Irritability 

 may be manifested by visible reactions, but it is probable that 

 responses to stimuli are for the most part invisible since they 

 have to do with activities and adjustments of the protoplasmic 

 substances rather than with the response of the organism as a 

 whole. 



The same environmental stimuli acting upon the fundamental 

 organization of a given species invariably produce the same reac- 

 tions and therefore result in the same type of derived organiza- 

 tion, so that the "species" remains constant. But if the environ- 

 ment be changed, then the stimuli from it become unusual, and the 

 protoplasmic response becomes more or less different from the 

 normal. If the change is too drastic, the protoplasmic substances 

 are unable to adjust themselves to a new equilibrium and dis- 

 integration results. Thus heat will speed up and cold will retard 

 activities, but too much heat coagulates the protoplasm. Chemi- 

 cals of different kinds introduce novel types of stimuli to which 

 the protopla.smic organization may adjust itself in such a way 

 that a different type of derived organization or an enduring modi- 

 fication results, a change that is maintained so long as the source 

 of the unusual stimuli remains in the environment. 



Obviously any change in the fundamental organization would 

 also result in a different series of reactions to the stimuli of the 

 normal environment. Thus, when, in fertilization, two cells fuse, 

 a new type of fundamental organization is produced which, with 

 the usual normal stimuli, may or may not be indicated by struc- 

 tural peculiarities of the derived organization. Should the fun- 

 * See "The Coming and Evolution of Life," page 6, in this Series. 



