Dr Alison on the Principle of Vital Affinity. 285 



it appears in the highest degree probable, that a gradual pu- 

 rification of the atmosphere by the agency of vegetables ab- 

 stracting carbon, was 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 state- 

 ments of Dr Draper), not with the heating portion of the rays, 

 nor with those which effect other chemical changes, but sim- 

 ply with the luminous portion of the rays, shews distinctly 

 that all living action on this globe is equally dependent on 

 light as on heat, althongli it is, and may long be doubtful, in 

 what manner the influence of light is exerted in producing 

 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 from any solid or liquid com- 

 pound containing it, or whether the agency of light may be 

 better expressed by saying, that it is the necessary stimulus 

 to that kind of vital action which leads to this primary trans- 

 formation of the elements of which organized beings are com- 

 posed. 



3. It is unnecessary to enter here on the varieties of this 

 amylaceous matter which are formed in different vegetables 

 or parts of the same, the cellulose of which the cells are formed, 

 the starch, tlie dextrin, the gum, the inuline, which are depo- 

 sited in different species and in different parts. All these 

 appear to have the same simple fundamental composition, 

 consisting almost entirely of carbon with the elements of 

 water, and all are formed out of the same compounds, and by 

 a vital affinity essentially the same ; it may be partly owing 

 to some imperceptible difference in the relative position of the 



