306 DR ALISON'S OBSERVATIONS ON 



which result from it, and of the period when it must have been first exerted on 

 the earth's surface, enables us to assert with confidence, that by means of it, 

 the whole organised creation has been, as Dumas expresses it, the offspring of 

 the air ; and that it was by enabling the rays of the sim to excite this action 

 in certain particles of matter, existing in the atmosphere, but destined to be 

 either the first specimens, or the first germs of vegetable life, that " a beneficent 

 God," to use the striking expression of Lavoisier, " has strewed the surface 

 of the earth, first with organized structures, and then with sensation and 

 thought/' 



In proceeding farther to inquire into the laws of Vital Affinity, we must 

 always keep in mind the general arrangement or classification, long ago made 

 by Dr Pkout, of all the organic compounds, of which any organized structures, 

 vegetable or animal, are composed, into three groups or classes, the Saccharine or 

 amylaceous, the Oily, and the Albuminous ; and the important observation, I be- 

 lieve first made by him, that the food of most animals contains all these com- 

 pounds, and that no complex animal structm-e can be maintained without the 

 concurrence of at least two of these kinds of compounds in its food. 



I do not think it is going too far to say that we have now a general know- 

 ledge of the laws or conditions under which all these compounds are formed in 

 living bodies, taking the starch formed from carbonic acid and water as the foun- 

 dation of all. But we perceive farther, that that these laws, varying in different 

 parts of the same structure, and at di/ferent times in the same parts, and being of 

 transient duration, in all, are liable to an influence of time and of place, and in 

 animals to a farther influence of mental changes, which is cpiite analogous to the 

 vital actions, both of muscular and nervous organs, but is strongly contrasted 

 with the uniformity of the laws that determine the changes of inorganic matter. 

 And if this be so, we may assert that considerable progress has been made, both 

 in establishing and in illustrating the doctrine of vital affinity, as a first principle 

 in physiology. 



I. The formation of Oil or Fat in living bodies is, perhaps, that part of the che- 

 mical processes there carried on, which is now the best imderstood, and the study 

 of which gives us the clearest insight into the nature of vital afiQnities. We need 

 not enter into any of the simply chemical questions as to the mode of combination 

 of the fatty acids and bases in the different kinds of fat ; it is sufficient for our pur- 

 pose to observe that they ai'e found very generally, though very variously disposed, 

 in almost all vegetables and animals, and even in the earliest stages of their ex- 

 istence ; the store of nourishment contained in the seed and in the egg, contain- 

 ing a proportion of fatt)^ matter. And though there is considerable variety in 

 the different kinds of fat or oil, they aU differ from the varieties of starch, by 

 having a much smaller proportion of oxygen, and, of course, a larger proportion 



