VIII A THEORY OF HEREDITY 285 



What Mr. Spencer calls polarities or polar forces 

 Haeckel speaks of as "undulatory movements" a 

 symbol which has the advantages and disadvantages 

 of analogy, but which, like "polarity," is only a 

 symbol, and covers our incapability of conceiving 

 more definitely the character of the phenomenon it 

 designates. The undulatory movement of the plasti- 

 dules is the key to the mechanical explanation of all 

 the essential phenomena of life. The plastidules are 

 liable to have their undulations affected by every 

 external force, and once modified the movement does 

 not return to its pristine condition. By assimilation 

 they continually increase to a certain point in size, 

 and then divide, and thus perpetuate in the undulatory 

 movement of successive generations the impressions 

 or resultants due to the action of external agencies on 

 individual plastidules. This is Memory. All plasti- 

 dules possess memory and Memory, which we see in 

 its ultimate analysis is identical with reproduction, is 

 the distinguishing feature of the plastidule ; is that 

 which it alone of all molecules possesses in addition 

 to the ordinary properties of the physicist's molecule ; 

 is in fact that which distinguishes it as vital. To the 

 sensitiveness of the movement of plastidules is due 

 Variability to their unconscious Memory the power 

 of Hereditary Transmission. As we know them to-day, 

 they may " have learnt little and forgotten nothing" 

 in one organism, "have learnt much and forgotten 

 much " in another, but in all, their Memory, if some- 

 times fragmentary, yet reaches back to the dawn of 

 life on the earth. 



