EXTERNAL CONDITIONS OF VITAL ACTIVITY. 59 



ditions of Vital Activity. Under the former group must be comprised, 

 not merely the Alimentary substances which are capable of being con- 

 verted into portions of the solid fabric, but also those which are used 

 (among the warm-blooded animals) for the maintenance of the bodily 

 heat by the combustive process. In addition, we have to include the 

 Water which is requisite to maintain the due proportion of liquid in the 

 organized fabric ; and the Oxygen, whose presence in the surrounding 

 medium is essential in various modes to the maintenance of its vital 

 activity. The dependence of Vital Activity upon Food and Oxygen 

 will be fully considered hereafter (CHAPS. IV. and VIII.); and in the 

 present Chapter it will be only necessary to take account of the demand 

 for Moisture (Sect. 4). 



75. The Forces to whose operation we can most clearly trace the 

 phenomena of Life, are Light and Heat, of which the latter is the one 

 whose agency is the most universal, and most immediately connected 

 with the acts of growth and development. The agency of Light is 

 indispensable for the first production of organic compounds by the in- 

 strumentality of the Vegetable fabric ; but it would possess no efficacy 

 whatever, without the simultaneous operation of Heat , and when these 

 compounds have been generated, we find that they can be applied to 

 the purposes of Vegetable nutrition, no less than to the nutrition of 

 Animals, without the aid of Light ; as is seen in the fact, that the ger- 

 mination of seeds takes place in darkness, and that the formation of 

 new wood in a stem takes place beneath a thick covering of bark. A 

 very large proportion of the vital operations of Animals have no direct 

 dependence upon Light ; yet it is entirely through its operation upon 

 Plants, that they derive the materials of their nutriment ; so that La- 

 voisier was fully justified in the assertion that " without Light, nature 

 were without life and without soul ; and a beneficent God, in shedding 

 light over creation, strewed the surface of the earth with organization, 

 with sensation, and with thought." As an example of the very direct 

 relation which subsists between the amount of Light and Heat acting 

 on an organism, and the amount of vital change produced, it may be 

 well to advert to the statement of Boussingault, that the same annual 

 plant, in arriving at its full development, and going through the process 

 of flowering and of the maturation of its seed, everywhere requires the 

 same amount of Light and Heat, whether it be grown at the equator or 

 in the temperate zone; the whole time occupied being inversely to the 

 intensity of these forces, and the rate of growth having a relation of 

 direct equivalence to it. We have little certain knowledge of the degree 

 of the ordinary dependence of Vital Activity upon Electricity; although 

 there can be no doubt that it is capable of exerting a most important 

 influence upon the living organism. 



76. In regard to all these Forces it may be observed, that the de- 

 pendence of Vital Action upon their constant influence is greater in 

 proportion to the high organization of their structure, and vice versd ; 

 so that beings of simple organization are capable of enduring a depri- 

 vation of them, which would be fatal to those higher in the scale. This 

 will be partly understood, when it is borne in mind that the higher the 

 development of the living being, the more complete is the distribution 



