204 METAMORPHOSIS OF TISSUE. 



frequently been attempted and carried out with more or less 

 success, and pains have been taken to trace through their minutest 

 modifications the differences presented by the specific methods of 

 combination occurring in the two kingdoms, and the peculiar 

 results of the individual forces manifested in these different spheres ; 

 but an exaggerated zeal for sharply defined distinctions of objects 

 has frequently led to assertions which have rather tended to retard 

 than advance the course of investigation. It almost seems like a 

 satire upon Liebig's thoughtful researches, when we find the 

 distinctions between the two kingdoms of nature laid down in such 

 a manner as the following: "The plant generates neutral non- 

 nitrogenous bodies, such as fats, sugar, starch, and gum ; decom- 

 poses carbonic acid, water, and salts of ammonia; developes oxygen ; 

 absorbs heat and electricity; is an apparatus of reduction, and is 

 immovable. The animal consumes neutral non-nitrogenous bodies, 

 such as fats, starch, sugar, and gum; generates carbonic acid, water, 

 and salts of ammonia; absorbs oxygen; developes heat and electricity; 

 is an apparatus of oxidation., and is movable.' 5 But nature will not 

 be restricted within such narrow bounds, and in the teeming rich- 

 ness of her forms and phenomena, she speedily burst the bonds with 

 which the human intellect capriciously attempts to restrain her. 



Many of these distinctions are applicable when considered only 

 in their most general bearings. The idea of a perpetual circulation 

 in nature is most forcibly expressed in these two series. It is 

 undoubtedly true that the organic matter which is generated in the 

 vegetable kingdom is for the most part again destroyed in animals, 

 but the idea that animals consume only protein-bodies, fats, and 

 carbo-hydrates, and cannot also in part generate them, is an assump- 

 tion which partly is false, and partly does not admit of proof. No 

 one can any longer doubt that the animal body possesses the power 

 of forming fat from other matters, such as protein-bodies or carbo- 

 hydrates (see vol. i, p. 255). It yet remains an open question 

 whether protein -substances may not also be generated in the animal 

 organism under certain conditions, although it is most probable 

 that such substances cannot be generated in the animal body 

 (vol. i, p. 346). If we except the lower animals, we certainly are 

 compelled to deny that the animal organism possesses the property 

 of forming starch and cellulose ; but sugar and dextrin are con- 

 stantly generated within the bodies of the herbivora during 

 digestion by the action of the saliva and pancreatic juice on the 

 other carbo-hydrates (see vol. ii, pp. 31 and 114 ) ; and even in the 

 bodies of the carnivora the liver has been recognised as a seat of 



