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- 
°Oscar Hertwig: Die Zelle und die Gewebe. I, P. 76. 
"Claude Bernard: Legons sur les phénoménes de la vie com- 
muns aux animaux et aux végétaux. P. 248, 281, 
