184 DR ALISON'S OBSERVATIONS ON 



1. We see this cliange effected, in the present order of things, only by the 

 agency of one of tlie amylaceous principles themselves, although the quantity of 

 that pre-existent matter, in the case of the seeds of many vegetables, is exceed- 

 ingi}^ minute. We need not enter on the question how far, besides the pre-exist- 

 ence of matter capable of forming cells, in the textures of the plant itself, previ- 

 ously existing organized matter, in the dead state, is essential as part of the nu- 

 triment of vegetables, — farther than to observe, that, as the seed of every plant 

 contains a store of organic compounds already formed, there is certainly a strong 

 presumption that a certain cpiantit}' of such compounds, formed by previous 

 living processes, is highly useful, if not necessary, to the nourishment of vege- 

 tables as well as animals. This, however, appears most important in the early 

 period of the existence of plants, when their power of decomposing the carbo- 

 nic acid has not yet attained its full intensity. The evidence of the greater 

 part of the nourishment of vegetables being from carbonic acid, water, and 

 ammonia, applied to their leaves, or absorbed by their roots, is quite conclusive ; 

 and when we consider that vegetables preceded the appearance of animals on 

 earth, that the first vegetables (as is well observed hj Liebig) were of the kind 

 which depend least on their roots and most on their leaves for subsistence, and 

 that the kinds of animals which first inhabited the earth, were those which con- 

 sume the smallest quantity of oxygen, and can live, therefore, in air highly 

 charged with carbonic acid, — it appears in the highest degree probable, that a 

 gradual purification of the atmosphere by the agency of vegetables abstracting 

 carbon, Avas a necessary prelude to the introduction of animals, especially of 

 warm-blooded animals, into the world : and that the greater part of the carbon 

 now existing in the soil on the earth's surface, originally existed in the form of 

 carbonic acid in the atmosphere, and has been gradually fixed, and enabled to 

 become the chief support of all living beings, by this vital affinity of vegetables, 

 and of those tribes of the lowest marine animals, which have been found to pos- 

 sess the same property, whereby carbon is separated from oxygen, and combined 

 with the elements of water, to form the amylaceous matters. 



2. The dependence of the exercise of this property on the presence of light, 

 and its connection (according to the statements of Dr Draper), not with the heat- 

 ing portion of the rays, nor with those which effect other chemical clianges, but 

 simply with the luminous portion of the rays, shews distinctly that all living 

 action on this globe is equall}' dependent on light as on heat, although it is, and 

 may long be doubtful, in what manner the influence of light is exerted in pro- 

 ducing this change ; whether the theory long ago proposed by Sir H. Davy is 

 admissible, that light enters into the composition of oxygen gas when disengaged 

 fi-om any solid or liquid compound containing it ; or whether the agency of light 

 may be better expressed by saying, that it is the necessar}' stimulus to that kind 

 of vital action which leads to this primary transformation of the elements of 

 which organized beings are composed. 



