298 Explanation of Inheritance 



acquire in this way, on account of the considerable mass 

 of this substance, a potential energy capable of over- 

 coming a considerable resistance to its discharge, and 

 would then be able to respond always in that single way 

 only which corresponds to the single specific nervous cur- 

 rent which it is able to activate and which constitutes its 

 irritability, even if it be provoked to discharge by external 

 influences or accidental stimuli which are quite different 

 from those to which it is ordinarily exposed. 



"A muscle cell," says Oscar Hertwig, "replies to every 

 kind of stimulus by contraction, a gland cell by secretion; 

 an optic nerve can perceive only light, no matter whether 

 it be stimulated by light waves, by electricity or by press- 

 ure. Similarly plant cells also are endowed with their 

 own specific energies : the reaction to stimulation re- 

 ceives everywhere its specific stamp from the special struc- 

 ture of the irritable substance, or in other words, 

 irritability is a fundamental property of living protoplasm, 

 but under the action of the environment manifests itself 

 in specific reactions according to the specific structure of 

 that protoplasm." 216 



And Claude Bernard defined irritability as : "the 

 property of living elements of reacting each according to 

 its nature to an external provocation or stimulus." 217 



"One conceives of the irritable substance," continues 

 Hertwig, "as a system of material particles in unstable 

 equilibrium, provided with forces at high tension. In 

 such a system, a very small shock of a single particle is 

 sufficient to put all the other particles in motion, each 

 transmitting its own motion to its neighbor. That ac- 



21fl Oscar Hertwig : Die Zelle und die Gewebe. I, P. 76. 

 217 Claude Bernard: Legons sur les phenomenes de la vie com- 

 muns aux animaux et aux vegetaux. P. 248, 281. 



