OF VITAL FORCE IN GENERAL. 51 



except when acted upon by Light ; and the amount of this change 

 which it effects, is in strict ratio (cceteris paribus\ with the illuminating 

 power of the rays which it receives ( 86). So, again, neither Plants 

 nor Animals can maintain their activity, except under the continual 

 influence of a certain measure of Heat ; and the amount of that activity 

 will be shown to bear a constant ratio, in all those tribes which have 

 no independent power of sustaining it, to the quantity which they receive 

 from external sources (CHAP. n. Sect. 2) ; this being true, not merely 

 of the general rate of the Vegetative actions of growth and development, 

 but also of those manifestations of vital power which are peculiar to 

 Animals. Thus we may say, that Light and Heat acting upon the 

 organic germ, become transformed into Vital force, in the same manner 

 as Heat acting upon a certain combination of metals becomes Electricity, 

 or as Electricity acting upon iron developes itself as Magnetism ; and 

 we shall find that this view is in complete harmony with all the pheno- 

 mena of Vital action. Moreover, the Vital force thus engendered fre- 

 quently manifests itself in producing Physical or Chemical phenomena ; 

 thus completing that mutual relationship, or correlation, which has been 

 shown to exist among the Physical and Chemical forces themselves ( 53, 

 54). Of this we have already seen an instance in the movements pro- 

 duced by muscular contraction and by ciliary vibration. The production 

 of heat by certain Plants and by warm-blooded Animals, is another appo- 

 site exemplification of the same principle. But the most remarkable 

 illustration is undoubtedly derived from the Nerve-force ; which, whilst 

 itself a peculiar form of the general Vital force, and capable of affecting 

 all the other manifestations of the same force (as in the modifications 

 which it produces in the processes of Nutrition and Secretion, as well as 

 in exciting Muscular Contraction), is capable of developing Electricity as 

 well as Light and Heat, and is also capable of being called forth by the 

 action of Light, Heat, Electricity, Chemical Affinity, or even Mechan- 

 ical Motion, on the Nervous tissue. It is a most remarkable confirma- 

 tion of the views here advanced, that the nerve-force, which must be 

 accounted, in its relations to Mind, as the highest of all the forms of 

 Vital force, should yet be the one which is most directly and intimately 

 related to the Physical forces, the " correlation" even of Electricity 

 and Magnetism not being more complete, than the "correlation" of 

 Electricity and Nerve-force may be shown to be ( 396). 



62. Thus, then, not only are the materials drawn from the Inorganic 

 world by vital agencies, given back to it again by the disintegration of 

 the living structures of which they form a part ; but all the forces which 

 are operative in producing the phenomena of Life, being first derived 

 from the Inorganic universe, are returned to it again under some form 

 or other. ^ The Plant forms those organic compounds, at the expense of 

 which Animal life (as well as its own), is sustained, by the decomposi- 

 tion of carbonic acid, water, and ammonia ; and the light, by whose 

 agency alone this process can be effected, may be considered as meta- 

 morphosed into the peculiar affinity, by which the elements of these 

 compounds are held^ together. The heat which Plants receive, acting 

 through their organized structures as Vital force, serves to augment 

 these structures to an almost unlimited extent, and thus to supply new 



