ON THE NATUKE OF LIFE. 183 



duced by this further compounding of highly com- 

 pound atoms have a more or less distinctive character. 

 We must conclude that in each case some slight differ- 

 ence of composition in these units, leading to some 

 slight difference in their natural play of forces, pro- 

 duces a difference in the form which the aggregate of 

 them assumes " (i. p. 182). We have here an indica- 

 tion of the great influence that may be exerted on the 

 ultimate varieties of power of the different kinds of 

 protoplasm by slight changes of composition of the 

 molecules. In the further development of the subject 

 he uses these physiological units chiefly to explain the 

 plastic faculty of living beings, viz., the innate ten- 

 dency of the living particles to arrange themselves 

 into the shape of the organism to which they belong, 

 "just as in the atoms of a salt there dwells the in- 

 trinsic aptitude to crystallize in a particular way." 

 This, for want of a better term, he proposes should be 

 called organic polarity. By these same units he ex- 

 plains also heredity and variation. " Sperm cells and 

 germ cells are essentially nothing more than the 

 vehicles in which are contained small groups of the 

 physiological units in a fit state for obeying their pro- 

 clivity towards the structural arrangement of the 

 species they belong to " (254). " The likeness to either 

 parent is conveyed by the special tendencies of the 

 physiological units derived from that parent." I do 

 not suppose these statements are put forth as explana- 

 tions, but their value lies in the admission that these 

 truly vital actions must belong to a substance pos- 

 sessed by living beings and vastly different from the 

 chemical proximate principles. Mr. Spencer, I per- 



