Factors in Evolution 25 



the germ, and are due to more or less marked diver- 

 gencies in the very constitution of the germ cells, 

 and these differences are reflected in the organisms 

 developed from them. How far these ontogenetic 

 variations are transmissible is hard to determine, 

 but it seems reasonable to suppose that they are 

 not without their effect in determining the future 

 history of the race. 



ENVIRONMENT 



Irritability. Protoplasm is distinguished by the 

 remarkable property of irritability, i.e., the power 

 of reacting to various external stimuli, such as light, 

 heat, electric currents, mechanical shocks, etc., as 

 well as to the so-called automatic stimuli, or those 

 arising within the protoplasm itself. These external 

 stimuli constitute an important part of the environ- 

 ment which is so powerful a factor in the shaping 

 of every organism. Were the protoplasm abso- 

 lutely uniform in all cases and the environment con- 

 stant, there would necessarily be no change, and 

 evolution could obviously not proceed. But by their 

 very nature the protoplasts of no two cells can be 

 exactly alike in structure, and there must be a cor- 

 responding degree of variability also in their be- 

 havior towards any stimuli. Moreover, the environ- 

 ment cannot remain absolutely constant but must 

 change to a greater or less degree. Inherent varia- 

 bility in the structure of the protoplast, and the 



