The Physiological Unit. 169 



inorganic matter, may, I conceive, be the special units 

 belonging to special kinds of organisms. By their 

 constitution they must have a plasticity, or sensitive- 

 ness to modifying forces, far beyond that of protein ; 

 and bearing in mind not only that their varieties are 

 practically infinite in number, but that closely allied 

 forms of them, chemically indifferent to one another 

 as they must be, may co-exist in the same aggregate, 

 we shall see that they are fitted for entering into 

 unlimited varieties of organic structures." * 



3. A further examination shows the physiological 

 unit to be composed of system upon system of mole- 

 cules, in successive degrees of complexity; each system, 

 and each combination of systems, having its peculiar 

 internal motions, and its individual polarity; and each 

 system having the equilibration of its polarity in the 

 perfected structure of the entire unit. "By combina- 

 tion of molecules with one another, and recombinations 

 of the products, there are formed systems of systems 

 of molecules unimaginable in their complexity. Step 

 by step, as the aggregate molecules so resulting grow 

 larger and increase in heterogeneity, they become more 

 unstable, more readily transformable by small forces, 

 more capable of assuming various characters."^ " The 

 chemical units combine into units immensely more 

 complex than themselves, complex as they are." \ 



* Biology, Vol. L, Appendix, p. 486. t Ibid., p. 486. 



I ^.,66. 



