206 GENEKAL EVOLUTION. 



Dr. Carpenter, in describing the correlation of physical and 

 vital forces, defines the difference of organic species to be similar 

 to that prevailing between different chemical bodies (the latter 

 depending on different molecular and atomic constitution), which 

 leads them " to behave differently " from each other under simi- 

 lar circumstances. This may be more fully expressed by saying 

 that different species possess different capacities for the location 

 of the conversion of the physical forces into growth-force. A 

 " descent with modifications " contemplated by a process of evolu- 

 tion, signifies a progressive change in this capacity. Acceleration 

 means an increase in this capacity ; retardation a diminution of 

 it. Grade-influence means the influence which produces this 

 change of capacity. 



B. The Origin of Grade-Influence. 



Living protoplasm can convert heat and nutriment into 

 growth-force without the agency of the nervous system. This 

 is proved by the nutrition of the Protozoa and Ccelenterata, and 

 from experiments on the muscles of frogs, etc. In the latter case, 

 as is well known, the nerve may be divided, and the muscle retain 

 its size if a current of electricity be passed through it, thus sus- 

 taining the nutrition. As the presence and structure of the nerv- 

 ous system is in relation to the specialization of animal struct- 

 ure in other respects, it is very probable that the nervous system is 

 in higher animals the agent of the location of growth-force. In 

 the lowest it is not affected by any such means. As the nervous 

 system is the instrument of the metaphysical peculiarities of the 

 animal (emotions, choice, etc.), we may conclude that, in the 

 lower animals, location of groAvth-force is influenced by necessity 

 without choice ; in the higlier by necessity with choice. 



The impulses derived from the nervous system, it is known, 

 may be reflex or automatic in answer to stimuli from without. 

 They may become so, also, after having been originated conscious- 

 ly or by effort of will. In the case of habits, frequent exercise of 

 choice has so impressed the nervous system as to result in its 

 repetition of effort, often in opposition to changed choice. 



The influence of effort in muscular action, through the nervous 

 system, appears to be, first, to enable it to convert heat to nerve- 

 force, and, then, to conduct nerve-force to the involuntary mus- 

 cles, or those controlling circulation, where it is converted into 

 motion, which thus controls nutrition through circulation. The 



