144 Dr Alison on the Prmciple of Vital Affinity. 



though we cannot explain the introduction of living beings 

 into the world, any more than we can explain the dissemination 

 of the stars throughout space, — although we must always re- 

 gard the appearance of organized bodies on the earth's sur- 

 face as the clearest indication which human knowledge pre- 

 sents of the subjection of the universe, not only to general 

 laws, but to an arbitrary Will, superior to these laws and 

 changing them at pleasure, — yet I think it may be said that 

 we have nearly as clear an insight into the designs and ar- 

 rangements of Providence for the maintenance of living beings 

 upon earth, and for the eternal reproduction of them there, 

 so long as these laws shall be in force, as we have into those 

 by which the movements of the heavenly bodies are directed 

 and controlled. 



1. Our first business is to study the facts that have been 

 ascertained in regard to the simplest form of chemical change 

 to which the term vital may be applied, which is merely se- 

 lection, by a portion of living structure, of some one substance 

 existing in a fluid, and the consequent attraction of this to a 

 particular part of the structure, while other materials, equally 

 presented to that living part, are excluded. 



We need not here enter into the question, on which che- 

 mists and agriculturists are not yet agreed, whether the 

 nourishment of plants, in the present condition of the earth's 

 surface, does or does not require the pre-existence, in the 

 soil, of organic compounds, resulting from previous living 

 beings, which are absorbed from it. But we may justly give 

 the name of vital attraction or affinity to that power by which 

 certain saline matters, dissolved in the compound fluid which 

 is absorbed, are retained in the substance of the plant, while 

 others are returned to the soil. " The experiments of Macaire 

 Princep,"*' says Liebig, " have shewn that plants, made to ve- 

 getate with their roots, first in a solution of acetate of lead, 

 and then in rain-water, give back to the latter all the salt of 

 lead which they had previously absorbed. Again, when a 

 plant, freely exposed to the air, rain, and light, is sprinkled 

 with a solution of nitrate of strontian, the salt is absorbed, 

 but is again separated by the roots, and removed farther 

 from them every shower of rain, so that at last not a trace of 



